function add_coach_id_field( $meta_fields ) {

    $meta_fields['coach-id'] = array( 'label' => 'Coach ID', 'type' => 'text', 'group' => 'leadsource' );

    return $meta_fields;

}

add_filter( 'wpf_meta_fields', 'add_coach_id_field', 10 );

function set_coach_lead_source() {

    if( isset($_GET[ 'coach-id' ]) && isset( $contact_fields[ 'coach-id' ] ) && $contact_fields[ 'coach-id' ]['active'] == true ) {
        setcookie( 'wpf_leadsource[coach-id]', $_GET[ 'coach-id' ], time() + DAY_IN_SECONDS * 90, COOKIEPATH, COOKIE_DOMAIN );
    }

}

add_action( 'init', 'set_coach_lead_source' );