Please open 1Password for Safari at least once and follow the directions being sure to allow it access to all sites. We need that permission to perform the following on your behalf.
The ability to scan (read) the page HTML to determine if it’s fillable. This also tells us where/when to inject the inline menu.
The ability to autofill (write) to the site. While performing an autofill, it effectively changes the HTML. When you submit the autofill, it will allow you to authenticate it properly.
There is no need to add 1Password for Safari to your login items or Dock. It only needs to be opened once for the initial configuration. From then on it can live silently in your Applications folder.