I write a custom plugin, with one link for administration purpose.
This is my code:

add_action('admin_menu', 'tb_instructor_menu');
add_action( 'init', 'aggiungi_ruolo', 11 );
function aggiungi_ruolo() 
{
$role = get_role( 'wdm_instructor' );
$role->add_cap( 'view_webinar', true );
}
function tb_instructor_menu()
{
add_menu_page('My code', 'My code', 'view_webinar', 'my-code', null, 'dashicons-businessman' , 61);
}

If I am logged in as Admin, I see the link.

If I’m logged in as “wdm_instructor”, I don’t see the link.

I have already verified that the “wdm_instructor” role has the “view_webinar” capability flagged.