Secondary recordset values in email result?

What is the best way to generate a value used in the final email, that requires a secondary post process based on user input (select list)?

For instance, there is a page with a form. A select list that has a Value (UserID) that is tied to a UserFirstName. The value is a simple integer. The goal is to get the UserFirstName for use in the email.

<option value="<?php echo($users->getColumnVal("UserID")); ?>"><?php echo($users->getColumnVal("UserFirstName")); ?></option>

I tried using the selected value on the email template page with a simple recordset ; select UserFirstName where id = $_POST['UserID']
Seems the email template is not the place for this.
Is the waue_....php file a good place to do the second recordset to use on the final email template?

Or should I be thinking a more javascript /jquery solution that works like AJAX to have the value needed present/generated as soon as the select list selection is made, then using a hidden input field have the value available via $_POST ?

Thanks you advance for the help.