Formidable Forms WordPress Plugin

For anyone looking for this as a fix I used frm_setup_new_fields_vars function instead:

add_filter('frm_setup_new_fields_vars', 'form_fullname_value', 10, 2);

function form_fullname_value($name_value, $field){
 //set array with all form fullname id's
$name_array = array(94, 83, 101, 185, 165, 114, 151);

//Set value for each fullname field
if(in_array($field->id, $name_array)){
//check if GET is set
if(isset( $_GET[ 'fullname' ] ))
  {
   $name_value['value'] = sanitize_text_field( $_GET[ 'fullname' ] );
   //write cookie
   setcookie("VNX[fullname]", $name_value['value']);
  }
else{ //see is cookie is set    
   if(isset($_COOKIE['VNX']['fullname']))
     { 
      $name_value['value'] = $_COOKIE['VNX']['fullname']; 
     }
  }  
}
return $name_value;
}