During the day, you usually leave your car down at the cable car station near Seis, which costs 17 Euros to get up and down or around 11 Euros for a one way ticket. You will arrive with the cable car at Compatsch and start your hike from this place or use the bus system of the alp to drive towards Saltria, which saves you a walk of around 45-60 minutes.
During the evening, you can drive up there. The street is closed from 9am to 5pm. Parking at the first parking place up there is free from 6pm, but it's another 15-20 minutes walk towards Compatsch. Parking at Compatsch itself costs 17 Euros for the day, no matter when you arrive and from 11pm the parking is not allowed to prevent campers from staying there.
Just make sure to not drive the roads to the hotels if you don't have a room there. The road is closed for anybody else and especially in the early morning and in the evening, they are making sure to check if you have a permission or not. Fines are around 112€ if you pass Compatsch and drive up there without having a room.