It's definitely possible, though you have to be careful about security.
If you install the Subscribe extra, you can include its unsubscribe.class.php file and use its utility functions in a snippet to generate a secure URL that sends them to a validation page. On that page, you can use another snippet that includes the unsubscribe class file again and use its functions to validate the user. Once you've validated the user, you can just do this:
$pageId = 12; /* ID of the page you're ultimately sending them to */
$user->addSessionContext('web');
$url = $modx->makeUrl($pageId, "", "", "full");
$modx->sendRedirect($url);
return;
Of course, you're free to create your own system for validating the user based on a hash sent as a parameter in the link, but I'd advise against using the user's password in that system. The Subscribe extra uses a hash involving the ID of the user's profile and their email, so if someone cracks it, the information can't be used to gain access to MODX.
