Since its been over a year since I last updated this specific code, I've done some quick refresher-reading on the topic.
The PayPal developer docs says:
By default, auto return is disabled.
And IIRC, as of UBB.threads 7.6.2, the return URL is sent to PayPal at the time of the transaction request. When the PayPal transaction is finished, that URL is used along with the PayPal results.
Basically, there is no need to enable "auto return" (or anything special) on your PayPal account, other than having a PayPal Business account and allowing payments to be accepted.
Prior versions of UBB.threads (7.6.1 and older) required PayPal "auto return" to be enabled since that portion of UBB.threads remained unwritten, and the user would just be dumped in to either a PayPal "thank you" page with zero links on it, or they would be returned to a broken UBB.threads page where they needed to contact the forum admin to manually update the subscription.