Can I auto-populate custom fields ?

In order to auto-populate a custom field defined for your membership subscription form you will need a custom script in the Values property of the field, for example:

//<code>
$db = Joomla\CMS\Factory::getDBO();
$db->setQuery("SELECT your_value FROM your_table");
$result = $db->loadResult();
return $result;
//</code>