<html>
<body>
<form action="/action_page.php" method="get">
Name: <input type="text" id="myInput" name="fname" required>
<input type="submit" value="Submit">
</form>
<script>
document.getElementById("myInput").oninvalid = function() {myFunction()};
function myFunction() {
alert("You must fill out the form!");
}
</script>
</body>
</html>