Extend Main Toolbar

To begin we need to register UICOMMAND() in YourModuleCommandsCommands.h

cpp

// YourModuleCommands.h
//   ...
public:
	TSharedPtr< FUICommandInfo > YourFirstCommand;
	TSharedPtr< FUICommandInfo > YourSecondCommand;

// YourModuleCommands.cpp

void FYourModuleCommands::RegisterCommands()
{
    // Example command with shortcut implementation
	UI_COMMAND(YourFirstCommand,
        "FirstCommand",
        "Desc of your first command",
        EUserInterfaceActionType::Button,
        FInputChord(EModifierKey::FromBools(true, true, false, false) /*control, alt, shift, command */, EKeys::T) 
    );


    // Example command without shortcut implementation
	UI_COMMAND(YourSecondCommand,
        "SecondCommand",
        "Desc of your second command",
        EUserInterfaceActionType::Button,
        FInputChord()
    );
}

Now you have to register theses commands in our module

cpp

// FYourModule.h
// ...
TSharedPtr<class FUICommandList> PluginCommands;


// YourModule.cpp
void FYourModule::StartupModule()
{
// requires
//#include "EditorStyleSet.h"
//Module: EditorStyle

// ...

    // Register our commands
	FYourModuleCommands::Register();
    
    // Map action and FUICOmmandInfoObject
	PluginCommands = MakeShareable(new FUICommandList);
	
    // Register YourFirstCommand
    PluginCommands->MapAction(FYourModuleCommands::Get().YourFirstCommand, FExecuteAction::CreateLambda([]{}));

    // Register YourSecondCommand
	PluginCommands->MapAction(FYourModuleCommands::Get().YourSecondCommand, FExecuteAction::CreateLambda([]{}));

    /** 
    * Add button to toolbar
    * If your custom can also run in commandlet
    * Exclude creation of buttons to avoid errors 
    */ 
	if (FSlateApplication::IsInitialized())
		AddButtonsToMainToolbar();
}


void FYourModule::AddButtonsToMainToolbar()
{
    // Owner will be used for cleanup in call to UToolMenus::UnregisterOwner(this);
	FToolMenuOwnerScoped OwnerScoped(this);
    
    /**
    * If you are using shortcut
    * you need bind our commandlist to LevelEditor side to handle shortcuts
    */
    // 
	FLevelEditorModule& LevelEditorModule = FModuleManager::LoadModuleChecked<FLevelEditorModule>(TEXT("LevelEditor"));
	LevelEditorModule.GetGlobalLevelEditorActions().Get().Append(PluginCommands.ToSharedRef());
    

    // Extend existing toolbar menu
	UToolMenu* Menu = UToolMenus::Get()->ExtendMenu("LevelEditor.LevelEditorToolBar.PlayToolBar");
	Menu->StyleName = "AssetEditorToolbar";

    // Add new section
    FToolMenuSection& Section = Menu->AddSection("Our new sections");
	Section.AddSeparator(NAME_None);

    // Create FirstCommandsEntry
    FToolMenuEntry YourFirstCommandEntry = FToolMenuEntry::InitToolBarButton(
		FYourModuleCommands::Get().YourFirstCommand,
		LOCTEXT("YourFirstCommand_Label", "YourFirstCommand"),
		LOCTEXT("YourFirstCommand_Tooltip", "YourFirstCommand"),
		FSlateIcon(FEditorStyle::GetStyleSetName(), "LevelEditor.GameSettings")
	);
    YourFirstCommandEntry.SetCommandList(PluginCommands);

    // now we can set the same style as ue5 button
    //  just a grey square, rounded left as background 
    YourFirstCommandEntry.StyleNameOverride = FName("Toolbar.BackplateLeft");

    // then register buttons  to our section
    Section.AddEntry(YourFirstCommandEntry);

    // do the same thing for the second buton
    FToolMenuEntry YourSecondCommandEntry = FToolMenuEntry::InitToolBarButton(
		FYourModuleCommands::Get().YourSecondCommand,
		LOCTEXT("YourSecondCommand_Label", "YourSecondCommand"),
		LOCTEXT("YourSecondCommand_Tooltip", "YourSecondCommand"),
		FSlateIcon(FEditorStyle::GetStyleSetName(), "LevelEditor.GameSettings")
	);
    YourSecondCommandEntry.SetCommandList(PluginCommands);
    YourSecondCommandEntry.StyleNameOverride = FName("Toolbar.BackplateRight"); // right background square
    Section.AddEntry(YourSecondCommandEntry);
}

Result: Awesome

Cheat Sheet

Generate button without UICommand

cpp

    FToolMenuEntry::InitToolBarButton(
        "",
        FUIAction(),
        LOCTEXT("MyButton_Label", "MyButton"),
        LOCTEXT("MyButton_Tooltip", "MyButton"),
        FSlateIcon(FEditorStyle::GetStyleSetName(), "LevelEditor.GameSettings")
    );

Add ComboMenuWidget to InitToolBarButton

cpp

void FYourModule::AddButtonsToMainToolbar()
{
    //...

    // Delegate to hide ComboMenuWidget by condition
    // (needed)
    FUIAction PlatformMenuShownDelegate;
	PlatformMenuShownDelegate.IsActionVisibleDelegate = FIsActionButtonVisible::CreateLambda([]()
	{
		return true;
	});

    // Create Entry
    FToolMenuEntry MyComboEntry = FToolMenuEntry::InitComboButton(
		"MyComboMenu",
		PlatformMenuShownDelegate,
		FOnGetContent::CreateLambda([this] { return MakeComboMenuWidget(PluginCommands.ToSharedRef()); }),
		FText(),
		LOCTEXT("MyComboMenu_Tooltip", "Settings related to my plugins.")
	);
    MyComboEntry.StyleNameOverride = FName("Toolbar.BackplateRightCombo");
	Section.AddEntry(MyComboEntry);
}

TSharedRef<SWidget> FYourModule::MakeComboMenuWidget(TSharedRef<FUICommandList> InCommandList)
{
    static const FName MenuName("UnrealEd.PlayWorldCommands.MyCustomComboMenu");
	
    // Ensur is not already register
    if (!UToolMenus::Get()->IsMenuRegistered(MenuName))
	{
        UToolMenu* Menu = UToolMenus::Get()->RegisterMenu(MenuName);
		{
			FToolMenuSection& Section = Menu->AddSection("MySettings", LOCTEXT("MySettings", "Settings"));
			Section.AddMenuEntry(FYourModuleCommands::Get().MyCommand,
                LOCTEXT("FirstCommand_Label", "First Command"),
                LOCTEXT("FirstCommand_Tooltip", "First Command tooltip"),
                FSlateIcon(FEditorStyle::GetStyleSetName(), "LevelEditor.GameSettings"));
            );
		}

        // we can also add Slate
        // for example wih a tip section
		{
			FToolMenuSection& Section = Menu->AddSection("Tips");
			Section.AddSeparator(NAME_None);
			Section.AddEntry(FToolMenuEntry::InitWidget(
				"Tip",
				SNew(SBox)
				.Padding(FMargin(16.0f, 3.0f))
				[
					SNew(STextBlock)
					.ColorAndOpacity(FSlateColor::UseSubduedForeground())
				.Text(LOCTEXT("Tip", "Lorem Ipsum tips"))
				.WrapTextAt(250)
				],
				FText::GetEmpty()));
		}
    }

    // Get all menu extenders for this context menu from the level editor module
	FLevelEditorModule& LevelEditorModule = FModuleManager::GetModuleChecked<FLevelEditorModule>(TEXT("LevelEditor"));
	TSharedPtr<FExtender> MenuExtender = LevelEditorModule.AssembleExtenders(InCommandList, LevelEditorModule.GetAllLevelEditorToolbarPlayMenuExtenders());
	
    // Bind commandlist to context
    FToolMenuContext MenuContext(InCommandList, MenuExtender);
	return UToolMenus::Get()->GenerateWidget(MenuName, MenuContext);
}

Result with ComboMenu: Awesome

References

ComboMenu: row 743 in DebuggerCommands.cpp
Extend Toolbar: row 579 in DebuggerCommands.cpp

Extend Main Toolbar ​

Create button and more in main Toolbar ​

Cheat Sheet ​

References ​

Extend Main Toolbar

Create button and more in main Toolbar

Cheat Sheet

References