zulyamodx,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script>
window.addEventListener('DOMContentLoaded', function() {
var form = document.querySelector('.form');
form.addEventListener('input', function() {
form.pagetitle.value = 'Заявка от ' + form.surname.value + ' ' + form.name.value
});
});
</script>
</head>
<body>
<form class="form" action="" method="post">
<input id="pagetitle" name="pagetitle" value="" /> <!-- для теста удалено type="hidden" -->
<span class="help-block">Фамилия</span>
<input id="surname" name="surname" type="text" value="[[+fi.surname]]" placeholder="" class="form-control input-md" required="required">
<span class="help-block">Имя</span>
<input id="name" name="name" type="text" value="[[+fi.surname]]" placeholder="" class="form-control input-md" required="required">
</form>
</body>
</html>