So I decided to build a simple RSVP for my wedding website using a formidable pro form. The trouble with that is obviously if people that aren’t invited to your wedding want to just RSVP then they can fill out the form and clog your inbox with false submissions. I then notice, ‘Hey, formidable pro comes with a password field’. I could just have a password on each of the invites so that only those that are invited can submit the RSVP form.

The only trouble is that the password field doesn’t really seem intuitive in the dashboard. What you will notice is that there isn’t really a ‘password’ to put into the field to validate. That apparently isn’t out of the box. The only way to add a password validation is to do it custom in the functions.php file. Below is what I use to validate the password field.

add_filter('frm_validate_field_entry', 'my_custom_validation', 10, 3);
function my_custom_validation($errors, $posted_field, $posted_value){
  if($posted_field->id == 85){ // Change this # to the ID of the field to validate
    if(!in_array($posted_value, array('passwordgoeshere'))){ // Change passwordgoeshere to whatever your password will be
    $errors['field'. $posted_field->id] = 'That field is wrong!'; // This is your error message
    }
  }
  return $errors;
}

Reference: Here