Match Field Input

I had asked this a while back and answer was provided, but I wasnt fully clear of it.
I have been trying for over a month to get this work and have yet to.

Im trying to have two fields that match. Such as form when the user has to enter their password two times and they must match. Instead im just changing the field name.

Maybe im doing it wrong and entering the code in the wrong area. I was told to put JS in the html after area. Im doing so on the seconded field.

The two fields im trying to match are:
phone and phone again

Code im using is:

<script type="text/javascript">
    function isNumber(evt) {
        evt = (evt) ? evt : window.event;
        var charCode = (evt.which) ? evt.which : evt.keyCode;
        if (charCode > 31 && (charCode < 48 || charCode > 57)) {
            return false;
        }
        return true;
    }
    function validateMe()
    {
    var phone = document.getElementById('field3_4').value;
    var phone_again = document.getElementById('field3_5').value;
    if(phone!=phone_again)
    {
    alert("Numbers are not same in both fields");
    return false;
    }
    else
    {
    alert("Yeah they match correctly!!");
    }
    }
    </script>

I have tried different codes and have yet to get anything to work.
Am I adding it in the correct area? Do you have some basic code I can put in there to get it to work or is there a matching passwords field I can use and just edit. Really lost on how to get this to work correctly.

Thank you in advance for any help

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