Can you teach me how to add a wallet or show me how to do it as an example? I get the message 'connect string isn't valid' all the time, maybe I still don't get it.

Go to nwc.getalby.com and create a nostr+walletconnect:... string there?

I put something like this, is it correct? (still not valid)

Something like this but it's not a correct link, where did you get it?

Remove pubkey= then it might work