Add a submenu page to Gravity Forms admin

I want to add a submenu page to Gravity Forms admin.
Tried to do it like this, but it's not working..

add_action('admin_menu', 'register_my_custom_submenu_page');
function register_my_custom_submenu_page() {
	add_submenu_page( 'gf_edit_forms', 'My Custom Submenu Page', 'My Custom Submenu Page', 'manage_options', 'my-custom-submenu-page', 'my_custom_submenu_page_callback' );
}

Which action should I use in the add_submenu_page function ?