I find that less important as you can make longer races in Free Play and therefore don't need to start as close to the front, but regardless, you can use the Car Groups to start you where you want. For overly complex things that already use them for multiclass and stuff it's tough to make it work right though.
Just set "Number of car groups" on the Rules page to 3, and turn on "Order by car group."
- If you want to start on pole, put yourself in Car Group 1, move all the AI to Car Group 2. Just do the opposite if you want to start last.
- If you want to start say 10th for example, put yourself in Group 2, and move 9 cars to Group 1, and the remaining cars to Group 3. Works for any grid spot.
I get that it's absolutely stupid that you have to do it that convoluted way when they could just copy and paste the thing from Career, or give us qualifying, but where there is a will there is a way, and working around the stupid limitations to enjoy the game has been something Forza Motorsport players have been needing to do for like 15 years now. As an added bonus you can use the Free Play settings to make the races longer, multiply the tire wear, add mandatory pit stops, customize your own weather, speed up time, etc so it really allows you to create more varied and engaging races.
You can also use the bucket system to make the AI a bit faster than you if you need a handicap. Just raise the PI in the 2 buckets you aren't in slightly above your bucket, and in most cases the AI will upgrade their cars... It can be a little janky though as sometimes they will refuse to upgrade without quitting out of Free Play and loading in again (so it recalculates the grid), and obviously some cars with limited upgrades (mostly certain race cars) won't work for it.
Here's to hoping when they
finally bring us manual grid for private lobbies (one of the devs said it was planned on the official forum) that it's also usable in Free Play.