You won't be able to, using only HTML, you'll either need javascript to dynamically change the "to" field, or some kind of server-side language, that can construct the email on the fly.
Either way it falls out of the realm of HTML and therefore of this forum.
You can try:
The javascript forum here: forum216
or for Server-Side options:
PHP forum: forum434
ASP forum: forum333
Cold Fusion Forum:232
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.