Relay is free and open source. Select the build for your system.
Move Relay to Applications. At the first start, macOS cannot verify the app. Open System Settings → Privacy & Security and click Open Anyway. Or clear the quarantine flag in Terminal:
SmartScreen shows “Windows protected your PC”. Click More info, then Run anyway.
Make the AppImage executable. Some distributions also need their FUSE package.
Source code: https://github.com/lubomirmolin/relay. Contact: hello@relaycode.io.