I booked an economy ticket on Air Canada's site. I'm flying from MXP, connecting in FRA to hop another plane going to YYZ. I'll be on Air Canada's codeshare partner Lufthansa when traveling to FRA and YYZ. I am not able to particularly choose a seat on the flights on route to FRA and YYZ. It says that it's not available at this time. I can't stand sitting in the middle and I prefer an aisle seat. Is there way, other than showing up at the airport super early, so I can pre-select a seat on the the Lufthansa flights?

thanks,