espltd,
<script>
function Inc20ut(obj)
{
test.value = test.value.replace(/@.*|$/, obj.value)
}
</script>
<input id="test" name="">
<select size="1" name="select1" id="select1" onchange="Inc20ut(this)" >
<option selected value="">выбор</option>
<option value="@m.ru">m.ru</option>
<option value="@1c.ru">1c.ru</option>
</select>