javascript form submition

Dear Experts,
I use a javascript to ask the user 'Are you sure?' before they submit the form

Below code works fine when my user clicks the submit button. But I want it to work just before the form is submitted. ( not onclick )

How can I do that? ( I know that I should use it on the <form tag like onSubmit  but I couldn't manage it)

<input type="submit" name="submit" id="submit" value="Submit" onclick="clicked(event)">

<script>
function clicked(e)
{
    if(!confirm('Are you Sure?'))e.preventDefault();
}
</script>

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