Forza Horizon 4ΒΆ
Enable UDP telemetry from HUD OPTIONS:
Start Forza Horizon 4
Enter Settings from the main menu
Goto HUD AND GAMEPLAY settings
Scroll to the bottom and enable DATA OUT
Enter your local IP address
127.0.0.1
as DATA OUT IP ADDRESSEnter the number
5607
as DATA OUT IP PORTExit the game

Tip
These steps are automatically verified and performed by SIMRIG Control Center. Start the game and follow the prompts issued by SIMRIG Control Center. Or use the auto-configure tool on the Settings Page.
Open a CMD window as Administrator and run the command
CheckNetIsolation.exe LoopbackExempt -a -n=Microsoft.SunriseBaseGame_8wekyb3d8bbwe

This will allow Forza to send UDP packets to your local computer. This is not allowed by default since Forza is packaged as a UWP app.
Done