After you file

IF...if you are actually asking about the 5-digit filing PIN, (and NOT the 6-digit IP-PIN)

The

If you log into your online account from last year, and download a NEW PDF copy of last year's tax file.

That big new PDF file will contain an Information Worksheet,  And on the last page (4) of that Info Worksheet is the 5-digit PIN you used last year.

 

Try that.

____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*