Forza Horizon 3 will be available through windows store only as far as we know.
This means you can go to storage settings and choose your SSD for apps locations. However as you know this means all app will install to your game drive. This is just the way the windows store works and they don't seem inclined to change it. So yes you can install it to your SSD but other apps will install there too. Most apps don't take up anywhere near as much room as games though, so don't worry. I have all my app and games from windows store install to my second SSD and it's fine as they live in the windows folder for that drive so it keep it tidy. It literally is for Forza Apex, couple of apps and then all my steam games etc install to the games folder on that SSD.
It's not ideal, but it's not too messy or irritating so i just ignore it. I brought this up repeatedly during the beta, as well as the problems with permissions that some users face, but i guess it's a small enough number of people for them not to bother.
Just be glad you can play Horizon 3 on PC with all the shiny shiny! :-)