The two Macs find each other on their own, without addresses to type. Since macOS 15 that needs the Local Network permission, also over a Thunderbolt cable, and macOS asks once on each Mac.

  • On the Mac you work on, setup explains it at Local Network: “Required to find the iMac, also over the cable.”
  • On the display Mac, allow it when macOS asks, so your other Mac can find this one.

If you clicked Don’t Allow

  1. Open System Settings › Privacy & Security › Local Network.

  2. Turn on Second Display. Check both Macs.

While access is off, the Mac you work on says “Local Network access is turned off for Second Display.”, and the display Mac says “Other Macs can’t see this Mac. Allow Local Network in System Settings.” Each message comes with a button to the right place in System Settings.

macOS 14 and earlier don’t have this permission, so there’s nothing to allow there.