/**

You need to hook into um_submit_form_errors_hook with a
priority over 10.

You can stop registration here if the customer_code field does not
match the value you want "ALLOWED_VALUE" in this example

*/

add_action('um_submit_form_errors_hook', 'check_customer_code', 100 );
function check_customer_code( $args ){

  if ( $args['customer_code'] != 'ALLOWED_VALUE' ) {
    exit( wp_redirect( add_query_arg('err', 'invalid_customer_code') ) );
  }

}