Good Night,
I am not able to join the Demo Version of Forza Horizon 4.
I always get this message, wich i posted in this link:
https://www.pic-upload.de/view-35956961/FotzaHorizon4Demobugg.jpg.html
I am looged in in xBox, Forza and Microsoft Homepage, and i start the Demo in the Microsoft Store in wich i am also logged in.
I need some help.
Thanks Guys 
You need to login inside the MS Store and the Win10 XBox App (with your existing MS account). Not on the websites.
You need to download the app from the store.
You may also be prompted to allow 'communication" between Win10 and the XBox App. You need to allow these.
Please try again.
The screenshot also shows that your Win10 is not activated. Maybe it has something to do with this, but I don’t think so. Please try the Xbox App login.