manipulating html forms

Hello,
Consider the following form
<form method="get" action="http://domain.com/sample.php">
Enter first part of your email<input type="text"
name="firstpart_email" value ="" size="20" />
<input type="submit" value="submit" />
</form>

If a user inputs "joe124", what code do i add in order for the form to
submit "joe124 (AT) user (DOT) mysite.com"