add_filter( 'rwmb_meta_boxes', 'user_profile' );
function user_profile( $meta_boxes ) {

    $terms = get_terms( 'custom_taxonomy', array( 'hide_empty' => false ) );

    $options = array();
    foreach ( $terms as $term ){
        $options[ $term->term_id ] = $term->name ;
    }

    $meta_boxes[] = array(
        'title' => 'Title',
        'type'  => 'user',
        'fields' => array(
            array(
                'name'       => esc_html__( 'Name', 'textdomain' ),
                'id'         => "_myid",
                'type'       => 'select_advanced',
                'options' => $options,
            ),
        ),
    );

    return $meta_boxes;
}