UserName.php
<form action="/user/admin/api/" method="POST">
<input type="text" name="name" value="<?=($for=='user_name'?$user->first_name:$user->last_name)?>" />
<input type="hidden" name="user" value="<?=$user->id?>">
<input type="hidden" name="api" value="User.update">
<input type="hidden" name="what" value="<?=$for?>">
<input type="submit" value="Submit">
</form>