Yes, you could set it when launching a game, but that’s temporary! At the end of the game it is being unset. To make it available for the current session use export NVPRESENT_ENABLE_SMOOTH_MOTION=1 or to make it permanent add it command to your .bashrc file.
On steam you can set launch parameters (launch options, where you paste commands per game). This is the one i am talking about. Since you say it is temporary i would think you talking about something else. Cause parameters you set in properties per game will stay there until changed so wouldn’t need to do anything. What i am basically asking is that if environment variables work there or if there is a specific location i need to enable them, example in Nvidia settings…..?