Comment by mtmail

You followed the correct reporting instructions.

https://www.fiverr.com/.well-known/security.txt only has "Contact: security@fiverr.com" and in their help pages they say "Fiverr operates a Bug Bounty program in collaboration with BugCrowd. If you discover a vulnerability, please reach out to security@fiverr.com to receive information about how to participate in our program."