for solution:

Automatically bind the challenge to a button

my application works as expected but

Programmatically invoke the challenge
token is always null.

simply I applied this answer:

/a/60067347/492258

<script src="https://www.google.com/recaptcha/api.js?render=mykey"></script>
<script>
  grecaptcha.ready(function() {
      document.getElementById('contactform').addEventListener("submit", function(event) {

        event.preventDefault();

        grecaptcha.execute('mykey', {action: 'homepage'}).then(function(token) {

           document.getElementById("googletoken").value= token; 
           document.getElementById('contactform').submit();
        });

      }, false);

  });
</script>

when I used ip such as
127.0.0.1 or prod ip
token returned is non empty string.

but on domain such as
localhost or prod domain
it is always null.