English version translated with Google Translate, some translation errors may occur.

First of all, the fact that it's not officially available is because they didn't feel like doing it (actually, I'm sure they wanted you to try the game with Game Pass and play it for a while, and then when you went to buy it on other platforms, you'd see that and go buy it officially there. These guys are smart, but no smarter than Fr33dan, the author of this discovery who makes everything so easy for us).
First of all, YOU MUST HAVE THE SAVED GAME FOR BOTH GAMES, both from Steam and XBOX. If you formatted the main hard drive where the XBOX files are saved, you have to reopen the game from XBOX to create a save or the one you had before. IF IT DOESN'T APPEAR THAT WAY, INSTALL BOTH GAMES ON DIFFERENT DISKS OR ON THE SAME ONE.

!-Open the GPSave Converter application. Git Hub link, latest update (no, I'll be waiting for updates, notice)

- https://github.com/Fr33dan/GPSaveConverter/releases/tag/v.0.4.7

2- You'll see 3 sections with 2 spaces on the right that say Xbox Save Files (Xbox saves) with another that says Non-Xbox Save Files below (saves that aren't from Xbox, in other words, Steam.)

3- If you already have the saves, the game should appear in the first section at the top left, in this case Forza Horizon 5. From there, go right, select it and that's it. If everything went well, you'll see that in the Xbox section there are a ton of things and that. In the Steam section, if you haven't played much or have played very little, you'll see few files. I STRONGLY RECOMMEND that you back up both saves. You'll only need to click on Explore Xbox Files and Explore Non-Xbox Files to bring them to the saves.

4- Click on the two down arrows between Xbox Files and Non-Xbox Files to automatically copy the files. Some errors will appear; click SKIP, nothing happens.

5- Now go to the Steam save files that were copied and search until you find a couple of files that start like this: User_901F8DD6D97CX. This is an example, but it's different for everyone. But that's the basics. You'll also see some that say "Xbox Users," I don't know what. They'll just copy the NUMBER WITH LETTERS, WITHOUT THE USER, so it says "XBOX," NOTHING ELSE. That's the only thing that will be changed.

6- After you've done everything, just open Xbox, log out, and open Steam FH5.

P.S. - If this didn't work for you, just delete all the data from the FH5 save folder on Steam. Open FH5, enter the game so that the new save folder is created, close it, and copy the Xbox files to that new folder and replace everything. (This is how it worked for me, since the other way didn't work for me, but it did for many other people.)