add script post when submit

Is it possible to add this code

<script type="text/javascript">

                  document.write('<form method=POST action="https://' + get_param("sip") + ':9997/login">');

                  </script>
                 <input type="hidden" name="username" value="anonymous">
                 <input type="hidden" name="password" value="anonymous" > 

                 </form>

in the form ?

I tried 2 ways :
1 – added in the page-myform.php inside
<div id="content" role="main">
2 – I created 2 custom fields
field label: username / field-type: hidden
field label: password / fiedl-type: hidden

both of them didn’t work ?

any suggestion?

https://www.ads-software.com/plugins/contact-forms/