https://www.bahn.com/en for German trains, search seat only

https://www.oebb.at/en/ for some German as well as Austrian and Italian, searching for tickets adding in Eurail pass as discount.

I don’t see where to add the global europass code

you add it as discout: Interrail/Eurail Global Pass

This is NOT as such a german train-with that we mean purely domestic. IF you want to RES and if its even possble (only on IC and ICE) then its daft easy via bahn.com - click on seat only. Cost 4,50, incl 2 if a transit is needed.

THESE INTERNAT trains -for those going TO and In IT there is a 10€ supplmt to pay for passholders. Will be collected to utter surprise and anger of many by conductor as last step if not done before.

IF you also want to RES-as such not mandatory, then you can do it also via bahn or for 3€ on oebb.at (austrian rails) as explained above. As so many times told here; if not done that, you run a tiny small risq of having to stand-perhaps only part of the way.

Note that there are just a very few of those train that run all the way to Firenze-others divert and go to Venezia or Milano. In that case change Verona and have to RES for an IT TrenITalia train onward.