I visited the Park&Charge on foot in March 2020.
There were many signs from the entrance directing customers to level 6 for the Park&Charge.
There was then a second ticket gate inside of the parking, where I think you had to enter your existing ticket. Then payment for the charging service was only time-based and would be added to your parking price.
I don't know whether the extra ticket gate is still there now that electricity is billed separately. I notice the system was Skidata, so they could use the Skidata EV platform instead of Swisscharge for unified billing in the future.