Implement reCAPTCHA v3 in ASP.NET and CodeBehind

I am trying to add reCAPTCHA v3 on my web page with C # .NET (code-behind) but I do not understand how I have to implement it on the server.

On the client I have the following:

Inside the HEAD

<script src='https://www.google.com/recaptcha/api.js?render=llavePublica'></script>

grecaptcha.ready(function() {
  grecaptcha.execute('llavePublica', {action: 'Accion'}).then(function(token) {
    // Verificar el token en el servidor.
  });

});

In 'llavePublica' I put the key of the domain that Google gives me when I created it in the reCaptcha admin account

In 'Action' I do not know what I should put

When I do a submit of the page with     Request.Form ["g-recaptcha-response"] I do not pick up anything, he shows it to me as null.

Neither do I know how to verify the token within the Javascript function.