Hi Blizz, thanks for reaching out!
If you're using the same single sign-in provider across these websites and they're a supported provider have you already tried to make use of the 1Password extension's "Sign in with" feature?
This feature allows users to save a sign-in provider in their Login items. 1Password can remember which providers you use for each website and sign you in with that provider, so that you don't have to enter a username and password each time.
If that doesn't work for your situation, let me know, and I'll be happy to submit a feature request to our Product team for the ability to link fields to another item.
I look forward to hearing from you!
-David