Apply discount automatically based on another membership

code.php

<?php

add_filter('ms_signup_payment_details', 'ms_signup_payment_details_cb', 10, 3);
function ms_signup_payment_details_cb( $invoice, $subscription, $membership ) {
	
	$current_membership_id = 1601;
	$discount = 30;
	$target_membership_id = 1343;
	
	if( $target_membership_id == $membership->id ){
		$member = MS_Model_Member::get_current_member();
		$membership_items = $member->get_membership_ids();
		if( in_array( $current_membership_id, $membership_items ) ) {
			$invoice->discount = $discount;
		}
	}
	
	return $invoice;
}