The following example will produce placeholders for the dropdown fields in a Credit Card field. Specifically, the Month and Year dropdown fields will read "Month" and Year".

add_filter('frm_combo_dropdown_label', 'add_cc_labels', 10, 2 );
function add_cc_labels( $label, $atts ) {
  if ( $atts['key'] == 'month' ) {
    $label = 'Month';
  } elseif ( $atts['key'] == 'year' ) {
    $label = 'Year';
  }
  return $label;
}