Add input checkbox dynamically to admin form in prestashop

I'm extending the employee page in admin prestashop to add an input text zip code and city checkboxs which i would to be created dynamically after entring the zip cod, i used a function to retrieve cities and create checkboxsbut it doesn't work

public function ajaxProcessGetVille(){
 $zip_cod = Tools::getValue('zip_cod');
 $html = file_get_contents("http://ift.tt/1BFEAgx".$zip_cod);
 $i = 0;
 $s = strlen($html);
 $htmlF = "";

 for( $i = 0; $i < ($s - 188) ; $i++){

  $htmlF = $htmlF.$html[$i + 186];

 }


 $c =  $json = json_decode($htmlF)->count;
 $f = "";

$options = array();                                 
for( $i = 1; $i <= $c  ; $i++){

$f = "".$i."";
 $ville = json_decode($htmlF)->$f->ville;

  $options[] = array(
        "id" => $ville,
        "name" => $ville
    );

}
//var_dump($this->fields_form);exit();
//$this->fields_form =$this->fileds_temp;

$this->fields_form['input'][]=  array(
                                      'type'    => 'checkbox',                         // This is an <input type="checkbox"> tag.
                                      'label'   => $this->l('Options'),                // The <label> for this <input> tag.
                                      'desc'    => $this->l('Choose options.'),        // A help text, displayed right next to the <input> tag.
                                      'name'    => 'options',                          // The content of the 'id' attribute of the <input> tag.
                                      'values'  => array(
                                        'query' => $options,                           // $options contains the data itself.
                                        'id'    => 'id_option',                        // The value of the 'id' key must be the same as the key for 'value' attribute of the <option> tag in each $options sub-array.
                                        'name'  => 'name'                              // The value of the 'name' key must be the same as the key for the text content of the <option> tag in each $options sub-array.
                                      ));
return parent::renderForm();

 //die($htmlF);
}

When i tested the form variable $this->fields_form it retrun null, before adding the checkbox adding code with $this->fields_form['input'][], any one have any idea and thanks.