Rewrite slug of custom post type to include post-id

Try this :

function change_post_type_link( $link, $post = 0 ){
    if ( $post->post_type == 'questions' ){
        return home_url( 'questions/'. $post->post_name ."https://wordpress.stackexchange.com/". $post->ID );
    } else {
        return $link;
    }
}

add_action( 'init', 'change_rewrites_init' );

function change_rewrites_init(){
    add_rewrite_rule(
        'questions/([a-z-]+)/([0-9]+)?$',
        'index.php?post_type=questions&p=$matches[1]&postid=$matches[2]',
        'top' );
}

Here post slug is attached after questions in the URL. For example questions/q-slug/45. Rewrite rule is changed accordingly.