pfSense

It can already be done by creating a custom login page with some javascript on it

<input type="text" id="realm" value="justADefaultRealm" onchange="update_user_field()" />
<input type="text" id="login" onchange="update_user_field()" />

<input type="hidden" id="auth_user" />

<script>
function update_user_field() {
    var realm = document.getElementbyId("realm").value;
    var login = document.getElementbyId("login").value;

 document.getElementbyId("auth_user").value = login+"@"+realm;
}
</script>

This issue can be closed.