When to digital fill entry forms (YUL-ZRH-OPO )?

I'm kind of freaking out needing to fill all those forms that we never had to fill before COVID. (this never happened before) and fearing something is missing.

I'm flying from Montreal (YUL) to Porto (OPO) via Zurich (ZRH) tomorrow.

I need to fill in the online registration to enter Portugal.

Portugal : "Additionally, all travelers must complete the online registration before departure"

It asks for the seat number; I still don't have the seat selected (web site does not allow it now).

Can I fill this form when I register and get my boarding pass at the airport ?

I assume that it should be good enough ?

Thanks.