Map ACF Relationship Field during Post Creation

I am trying to map an ACF Relationship field during the post creation process. The field is set-up to allow only a single selection. For every other field I can simply use something as follows:

add_action('gform_advancedpostcreation_post_after_creation_7', function ($post_id, $feed, $entry, $form) {
    update_field( 'field_63d65a709cc93', rgar( $entry, '17' ), $post_id );
}, 10, 4 );

However this approach does not appear to work for Relationship fields. Can someone please put me right?!