It’s always a challenge to keep these remote rooms warm. You’re fighting not only the room from losing heat due to the added surface area exposed to the exterior, but you’re fighting getting heat to the furthest place (usually) from the furnace. It would take a bit of experimenting with the duct vents but you’d have to close/limit the vents along the duct-work leading to that room and then leave the vents in that room wide open.
Or you could give up like I did and just run a small electric ceramic heater in the room to keep the temp up.