To Get this to work you will need to replace the Text “Team Leader” with the name of the select field you wish to populate. Everything else is a Basic wordpress Query

add_filter( 'wpt_field_options', 'populate_select', 10, 3);
function populate_select( $options, $title, $type ){
    switch( $title ){
        case 'Team leader':
            $options = array();
			$args = array(
                'post_type'        => 'team-leader',
                'post_status'      => 'publish',
				'meta_query' => array(
        							array(
							            'key'     => 'wpcf-active',
							            'value'   => 'Yes',
							            'compare' => '=',
        								),
									),
								);
            $posts_array = get_posts( $args );
            foreach ($posts_array as $post) {
                $first_name = get_post_meta($post->ID,'wpcf-first_name');
                $last_name = get_post_meta($post->ID,'wpcf-tmsurname');
                $options[] = array(
                    '#value' => $post->ID,
                    '#title' => $first_name[0]." ".$last_name[0],
                     );
                }
               
                break;
}
    return $options;
}