Hello Roizoaquoo79
You can make reservations on bahn.de and select the seat (6,50 €). On oebb.at you can indicate they that you want a panoramic car, but not select a specific seat (3 €).
You must add “Mainz” as via to get the EC and not an ICE.
Please note that you can choose another free seat if available, you have no obligation to stay on the reserved seat.
Kind Regards
Danhiel