[redhead]

<script language="JavaScript">
function populate(text)
{
     document.my_form.email.value = text + "@domain";
}
</script>
<form name="my_form" action="what_ever" method="post">
<input type="text" name="name" value="" onUnFocus="javascript:populate(this.value)"> 
<input type="password" name="password" value=''>
<input type="text" name="email" value="">
</form>

Should give you an idear.