Bonus credits from Forza hub not transferring to game on PC version

If you mean the rewards from the Forza Hub then 1,500,000 credits is split across all Forza games,not 1,500,000 credits just for Forza 6.

The higher your tier the more credits you get per week if you redeem them every Friday.

The Forza Hub issue is with the Windows 10 app. On the console it works fine and it logs me in instantly. With the PC version, I get the spinning loading circle for 10-20 seconds, then it says that the Forza Hub is currently unavailable.

Sorry to hear that, Your friend will need to obtain the credits to purchase the car again, there are two ways of doing this, firstly earn credits in game as per usual, the second is to download the Forza Hub from the Xbox store. The Forza hub rewards a player
for completing certain in game feats, these are things like, earning achievements, completing PR stunts, discovering roads, etc. completing these feats, or making progress will earn a player Forza Hub points, this
page will explain the Forza hub rewards in much more detail, the post refers to them as Forza rewards, be sure to redeem your rewards every week

There's also the Forza Hub app, which gives out free Credits on several Forza franchise titles!

you can only import a file from forza 3 like you have already done.

this rewards you with bonus cars based on your level in forza 3.

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you cannot transfer anything else other than this.