<script>
function yazdir(id, veri){
document.getElementById(id).innerHTML=veri;
}
</script>
Alan adını seçin:<br>
<input type="text" onkeyup="yazdir('domain', this.value);">
<select name="uzanti" onchange="yazdir('uzanti', this.value);">
<option value="com">com</option>
<option value="net">net</option>
<option value="org">org</option>
</select><br><br>
Mail adresleri:<br>
<input type="text">@<span id="domain">deneme</span>.<span id="uzanti">com</span>