Snippet to add your own actions inside Unreal's main toolbar

Unreal Engine - Extend main toolbar

void FYourModuleModule::StartupModule()
{
	UToolMenu* MainTabFileMenu = UToolMenus::Get()->RegisterMenu("MainFrame.MainTabMenu.File", "MainFrame.MainMenu.File");
	FToolMenuSection& Section = MainTabFileMenu->AddSection("MySection", INVTEXT("My Section Name"));
	Section.AddMenuEntry(
		FName("MyAction"),
		INVTEXT("My Action Name"),
		INVTEXT("My Action Tooltip"),
		FSlateIcon(FName("OutputLogStyle"), "Log.TabIcon"),
		FUIAction(FExecuteAction::CreateLambda(
			[]()
			{
				UE_LOG(LogTemp, Warning, TEXT("MyAction was executed"));
			}
		))
	);
}