How to add input field for PayPal subscriptions?

[Servo]

Does anyone know a way that I can add an input field to my paid subscriptions when using PayPal? Basically I'm trying to give a custom field for the subscription buyer to add his or her xbox gamertag automatically into the PayPal notes.

So adding the javascript somewhere:

<script type="text/javascript">
function checkGT(){if(document.paypal.os0.value=='Enter Gamertag'){alert("Please enter your gamertag before buying!");return false;}}
</script>

And then adding the somewhere as well:

<form action="https://www.paypal.com/cgi-bin/webscr" target="_blank" method="post" onSubmit="return checkGT()" name="paypal">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="WHATEVER">
<table>
<tr><td><input type="hidden" name="on0" value="Enter your gamertag"></td></tr><tr><td><input value="Enter Gamertag" class="bullshit" size="15" maxlength="15" onFocus="if(this.value=='Enter Gamertag'){this.value='';}" onBlur="if(this.value==''){this.value='Enter Gamertag';}" type="text" name="os0" ></td></tr>

I think my biggest problem is that I don't know where exactly to add it...any clues?

Edit: or even in the subscription_payment_paypal template perhaps there's someone who knows how to add it there? I'm so clueless today.