https://www.google.com/recaptcha/admin/site/433910553 ahí los das de alta y consigues la api key y secret
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Recaptcha 2

<form method="post" onsubmit="return submitUserForm();">

    <div class="g-recaptcha" data-sitekey="6LcZ89wZAAAAAIBtihR10lCfpZOk218D2EkDaDIy" data-callback="verifyCaptcha"></div>

    <div id="g-recaptcha-error"></div>
    
    <input type="submit" name="submit" value="Submit" />
</form>

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

<script>

function submitUserForm() {
    var response = grecaptcha.getResponse();
    if(response.length == 0) {
        document.getElementById('g-recaptcha-error').innerHTML = '<span style="color:red;">This field is required.</span>';
        return false;
    }
    return true;
}
 
function verifyCaptcha() {
    document.getElementById('g-recaptcha-error').innerHTML = 'Valid Captcha';
}
</script>

Recaptcha 3, requiere manejes la respuesta en PHP o hagas el código para js...



<script src="https://www.google.com/recaptcha/api.js?hl=en&render=6LeniNwZAAAAAOckY9y8v_3iMDI4a04qV3SbY9Vz"></script>

<form id="enq-frm" role="form" method="post" action="#" class="form-horizontal">
    <input type="text" class="form-control" id="fname" name="fname" placeholder="Full Name">
    <input type="email" class="form-control" id="uemail" name="uemail" placeholder="Email Address">
    <textarea class="form-control" id="msg" name="msg" placeholder="Enter your message" rows="5"></textarea>
    <input type="submit" value="Post" class="btn btn-success btn-block" name="post">
    <input type="hidden" id="token" name="token" value>
</form>

<script>
    grecaptcha.ready(function() {
        grecaptcha.execute('6LeniNwZAAAAAOckY9y8v_3iMDI4a04qV3SbY9Vz', {
            action: 'homepage'
        }).then(function(token) {
            console.log(token);
            document.getElementById("token").value = token;
        });
    });

</script>