TLDR: Length extension attack on SHA1. Use `https://github.com/stephenbradshaw/hlextend` to extend the known msg. Since key length is unknown, iterate through keylengths from 10 to 109 until we hit a match. Use that key length to submit the forged message. 
https://meashiri.github.io/ctf-writeups/posts/202306-nahamcon/#forge-me-2