## inside my form tags...
<div class="formwrap">
  <label for="user_email_address">First Name:</label>
  <input class="mandatory" id="user_first_name" name="user[first_name]" onkeydown="formAdvance(this);" size="30" type="text">
</div>

<div class="formwrap">
  <label for="user_email_address">Last Name:</label>
  <input class="mandatory" id="user_last_name" name="user[last_name]" size="30" type="text">
</div>

## external js file:

function formAdvance(el) {
	$(el).down('input').value = "test";
}