How password autofill works
Store credentials autofill on supported sign-in pages. On some pages, click a field and choose an account from the password chooser.
The Username and Password you enter when adding a store stay with that store. After you launch the store browser, they autofill on the sign-in page, so you don't copy and paste. See How to add a store.
Some sign-in pages don't autofill right away. Click the username or password field and the password chooser appears. Pick an account and both fields fill.
If the store has more than one saved login, click the field and pick from the chooser.
Don't sign in by importing cookies from another browser. See Why cookies can't be imported.