My kids will start playing Minecraft on a self-hosted server soon, but the newly purchased Java edition of Minecraft for Windows has the “multiplayer” button disabled. There’s a tooltip that says, “Multiplayer is disabled. Please check your Microsoft Account settings”. That would have been a helpful tooltip if there were any relevant “Microsoft Account settings” that can enable multiplayer. But there is no such setting! 
Google shows that this appears to be a general problem, and the suggested solutions do not work. Some posts even claim that this has been broken for so long that it’s outrageous that Microsoft still hasn’t fixed it. I can now confirm that there is a working solution, but it’s so obscure that not even an hour on the phone with first Xbox Support and then Microsoft Support was able to resolve it!
So, here’s the solution:
