Note
Original (now deleted / privated) repository: https://github.com/antunnitraj/Prism-Launcher-PolyMC-Offline-Bypass.
This bypass seems to have been fixed in PrismLauncher 10, but works with versions 9.4 and below.
Bypass check if user has a real Microsoft account linked with the launcher
Prism Launcher added a check of Online Accounts. The fix is to download the older version of Prism Launcher 9.4