This question has been answered by multiple community members. See the first response.

$usernameField = $this->modx->getOption('usernameField', $scriptProperties, 'username');
$username = $hook->getValue($usernameField);
$user = $modx->getObject('modUser', array('username' => $username));
if ($user) {
$userId = $user->get('id');
$profile = $modx->getObject('modUserProfile', array('internalKey' => $userId));
if ($profile) {
$fullName = $profile->get('fullname');
$modx->setPlaceholder('fullname', $fullName);
}
}
return true;
&postHooks=`FullName` &preHooks=`FullName`
[[+fullname]]