I am sorry to resurrect this thread, but I cannot get it to work.
When I start Minecraft with the normal Minecraft launcher, I choose Microsoft login and see my XBox gamer tag and am greeted with my XBox name. A click on the Java Edition starts the game.
When I try this with the FTB launcher instead, it goes like this:
— click on FTB, the apps opens, and I choose for example on Regrowth
— the normal MC launcher opens, and it shows up on the left side 'Player Mojang-Konto'
But here is only the Demo Mode visible. There is no way to log in now.
I CAN still login with a browser to Mojang, but there is no longer any mention that I ever possessed Minecraft at all. And I am not sure, where to see that at all.