Impossible to create a new login for a website directly from autofill screen ? Even if a login is already saved (when you have multiple accounts for example)
Not in the current beta, possibly in a future one.
+1 for this addition
This is currently possible in the release version. In the AutoFill toolbar you choose the 1Password… option and at the top of the 1Password list is "Create Login". It will create that login and upon completion it will fill that login into whatever form you're working with.
Sorry, you're correct I'm getting mixed up with another function..
I'd like to select a pre created login for a website that may use an alternative URL.
I know I can add the URL within the app first, but this is clunky and I'd like to be able to select it in the auto fill prompt and add it that way, for a more friendly UI experience.
If you mean you want to manually edit an existing login to add a new URL, you would need to do that from within the main 1Password app in that case. As illustrated in your screenshot, iOS 12 Password Autofill (and 1Password itself) will only show you logins matching the current URL. Otherwise it would be easy to mistakenly fill credentials into the wrong place -- like a phishing site/app.