A simple inline JavaScript confirm would suffice:
<form onsubmit="return confirm('Do you really want to submit the form?');">
No need for an external function unless you are doing validation, which you can do something like this:
<script>
function validate(form) {
// validation code here ...
if(!valid) {
alert('Please correct the errors in the form!');
return false;
}
else {
return confirm('Do you really want to submit the form?');
}
}
</script>
<form onsubmit="return validate(this);">