Unable to read selected value from List in the Joomla form - Joomla! Forum - community, help and support

hi,

i trying build plugin form fields. fields coded in usual mvc pattern. 1 of these fileds list defined follows -

<field name="aptblock" type="list" default="z"
description="com_users_register_aptno"
filter="string"
label="com_users_register_aptno_label"
message="com_users_register_aptno_message"
required="true"
size="1">
<option value="a">a</option>
<option value="b">b</option>
<option value="c">c</option>
<option value="d">d</option>
<option value="e">e</option>
<option value="f">f</option>
</field>

one of requirement tweak field in memory based on input in list above. trying in method - function oncontentprepareform($form, $data) - in plugin.

my question - how retrieve selected value of field in plugin -
$form->setfieldattribute('username', 'default', $form->getfieldattribute('aptblock', '<what should if have here>', 'a1', ''));

any highly appreciated.