frm_field_classes

Add classes to an input field.
Parameters: $classes (string), $field (array)

In version 1.6.3, this can be done without custom code, by going to edit your customizable HTML, and changing [input] to [input class="my_class"].

Examples

add_filter('frm_field_classes', 'add_input_class', 10, 2);
function add_input_class($classes, $field){
  if($field['id'] == 25){ //change 25 to the ID of your field
     $classes .= ' my_class';
  }
  return $classes;
}