wpmem_mc_merge

add_filter( 'wpmem_mc_merge', 'my_mc_merge_filter' );
 
function my_mc_merge_filter( $merge_vars ) {
 
    /*
     * The default merge_vars, first and last name come in
     * with the $merge_vars array.  If you are adding to
     * the merge fields IN ADDITION TO those, you'll need
     * to consider that as you filter the array.
     *
     * Here is an example where I keep the existing merge
     * fields (first and last name) and add two additional
     * fields to that array. Note how I obtain the values
     * from the posted form data.
     */
     
    $my_new_field = array(
        'MC_MERGE_FIELD_1' => $_POST['wp-members-field-meta-key-1'],
        'MC_MERGE_FIELD_2' => $_POST['wp-members-field-meta-key-2'],
    );
     
    $my_new_merge = array_merge( $merge_vars, $my_new_field );
     
    return $my_new_merge;
}