How to activate D-BOX motion in Assetto Corsa Competizione?

Step 1:

From the Assetto Corsa Competizione (ACC) menu, open the "OPTIONS" menu in the left bottom corner :

Step 2:

Inside the "OPTIONS" menu, go to the "GENERAL" submenu inside the "Gameplay" category:

Step 3:

Set "D-BOX MOTION" as "ENABLED" (3rd option for the top):

Step 4:

Save the changes and restart Assetto Corsa Competizione. Once restarted, the D-BOX motion should be functional.