Hi, i need an automatic subscription for some forums, for example id 3 and 4 for a userid

if want to add a function like this

function my_um_after_user_is_approved( $user_id ) {
    WPF()->sbscrb->subscribe( $user_id, 3);
    WPF()->sbscrb->subscribe( $user_id, 4);
}

but i didn't find a function for this. Is there a hook for this ?