How do I...add Widgets on Custom Launcher

Hello everyone, i am a new programmer who is building a custom Launcher with a team.
In the tutorials about widgets I learned how to create a new widget.
In my custom Launcher i would like to open the WidgetManager (or WidgetTab) with OnLongClickListener.
Is there a way to open them and then put the in screen ???
As i saw every Launcher has its own way to open it and i cant find the way...