Passwords for such files are usually set by the individual or site that uploaded the archive. If you downloaded this from a specific website, you may find the "paper" (password) in the following locations:
The password for the experimental controller beta branch on Steam is controllertest . Starcom.Unknown.Space.rar
Be cautious when downloading .rar files from unofficial sources, as they may contain malware or "password-protected" traps designed to make you complete surveys. Passwords for such files are usually set by