Welcome to the forums, Wolfgeek!
I split your post from the thread in the Chrome forum. I hope you don't mind, but the Chrome extension is a different beast, so the tips and/or troubleshooting procedures are a bit different at the moment.
That said, I would love to get your Plesk login working for you in Safari. Please try saving the login manually like this:
1. Enter your username and password, but DO NOT submit the form.
2. Click the 1Password button in your browser's toolbar, and choose Save Login.
3. Set the Action field to Create New Login (or to Replace <login-name>, to update an existing Login item).
4. Click the Save button.
Using the above technique I was able to save and fill a sample Plesk login in Safari.
Manually saving a login can be useful for logins that are either problematic to begin with or were once working but have since stopped. Doing this allows 1Password to refresh everything it "knows" about the page. Login pages often change as websites are updated and this can be a necessary but very useful tip.
If you are still having trouble, please take a look at our "Website Login Issues" guide:http://help.agile.ws/1Password3/login_issues.html
Note especially that editing the login to change the Submit value from "If Autosubmit is ON" to "Never" works on a lot of sites which modify the form before submitting it or are looking for a literal key press or mouse click. This will give you the chance to actually give them what they are looking for by submitting the form yourself rather than allowing 1Password to do it on your behalf.
I hope that helps. Please let me know. If you are still having trouble could you please provide the URL?
I followed your instructions and it still doesn't work. It doesn't automatically submit the login info. If I click on the 1P button and search for the login and click it, it submits the login info, but returns to the login page again. Logging in manually works fine.
Here is a video of me following your directions and showing the behavior of 1Password / Plesk / Safari once the login has been saved.
IT Director - ISP Operations
I turned off Autosubmit and it works now.