example of using ajax with user list

[Jimmi08]

Hi,
I have the field with the user list, after selecting some user, I need to fill another field with user name.

Could you give me an example of code on how to use this with admin UI?

User field:
'user_id' => array('title' => LAN_USER, 'type' => 'dropdown', 'data' => 'int', 'readParms' => [], 'writeParms' => ['default' => ''] ),

		$users = e107::getForm()->userlist('aid', null, array('return' => 'array'));
		$this->fields['user_id']['writeParms']['optArray'] = $users;

'ajax' => true should be used but what next? How to set that other field to display a new value?

Thank you

[Jimmi08]

EDIT
changed field to

		'user_id' =>  array('title' => LAN_USER,
		'type' => 'user', 'data' => 'int', 'width' => '5%', 'help' => '', 'readParms' => array(), 'writeParms' => ['default' => 99999999], 'class' => 'left', 'thclass' => 'left',),

userlist() is limited to 1000 users only.