Shell - How add programmatically sub menu item in flyout menu.

Hi, how add sub menu item in flyout menu programmatically?

My goal:

-MainMenuElement-01
-------SubMenuElement-01
-------SubMenuElement-02
-------SubMenuElement-03
--------------...
-MainMenuElement-02
-MainMenuElement-03

I can add programmatically MainMenuElement, but I can't add SubMenuElement in MainMenuElement.
I will be grateful for your help.