System 1

<form id="demo-form" action="">
        <input type="text" name="name">
        
        <button class="g-recaptcha" 
        data-sitekey="6LerAisqAAAAADRGg_XO1VzPhjuBK0Quthm-98ns" 
        data-callback='onSubmit' 
        data-action='submit'>Submit</button>
    </form>
    6LerAisqAAAAADRGg_XO1VzPhjuBK0Quthm-98ns
    6LerAisqAAAAAIkRU5noPqLr8aAwE127340dVP6l

    
    <script src="https://www.google.com/recaptcha/api.js"></script>
    <script>
    function onSubmit(token) {
        document.getElementById("demo-form").submit();
    }
    </script>

System 2

<form id="myForm" method="POST" action="submit.php">
  <!-- Your form fields here -->
    <input type="" id="recaptchaToken" name="recaptchaToken">
    <button type="submit">Submit</button>
    </form>


    
    <script src="https://www.google.com/recaptcha/api.js?render=6LerAisqAAAAADRGg_XO1VzPhjuBK0Quthm-98ns"></script>

    <script>
        function executeRecaptcha(action) {
            grecaptcha.ready(function() {
            grecaptcha.execute('6LerAisqAAAAADRGg_XO1VzPhjuBK0Quthm-98ns',{action: 'submit'}).then(function(token) {
                // Add the token to your form or send it to your server
                document.getElementById('recaptchaToken').value = token;
                document.getElementById('myForm').submit();
            });
            });
        }

        document.getElementById('myForm').addEventListener('submit', function(event) {
            event.preventDefault();
            executeRecaptcha('submit_action');
        });
</script>

Referance google recaptcha