That is good to know if the real time access to TSA from the airline computers has become reliable enough to do that just before issuing BP. I am dating myself but there was a time when you didn’t get the PreCheck unless it was done in some batch processing earlier and avoided the dreaded Ss which meant additional security checks. Have always entered it ahead since then.

The TSA recommendation is still to enter it at time of booking. Almost every “manage your booking” service for airlines and travel agencies allows this entry ahead at any time during or after booking.