Confirm before a form submit

<input type="submit" onclick="return confirm('Are you sure you want to do that?');">

HTML:

<form action="adminprocess.php" method="POST" id="myCoolForm">
    <input type="submit" name="completeYes" value="Complete Transaction" />
</form>

JavaScript:

var el = document.getElementById('myCoolForm');

el.addEventListener('submit', function(){
    return confirm('Are you sure you want to submit this form?');
}, false);

Edit: you can always use inline JS code like this:

<form action="adminprocess.php" method="POST" onsubmit="return confirm('Are you sure you want to submit this form?');">
    <input type="submit" name="completeYes" value="Complete Transaction" />
</form>