How can I set 1Password to fill an arbitrary field with a saved value?

@franklaw 1Password does indeed go through the HTML on all sites on a case-by-case basis in order to save and fill. For the website that you shared, I can see that 1Password doesn't recognize the username field. I've reported the field and website to our developers. Hopefully, we'll be able to adjust some things on our end to improve the saving and filling experience there.

For now, I recommend setting up the login item in the following way:

1. Open the 1Password app and click 'New Item' > 'Login'.
2. At the top of the login, make sure that the username field is blank.
3. Fill in the password field (for now).
4. Add 'https://seicashmanagement.mybankingservices.com' to the Website section.
5. Click 'Add More' and select 'Text'.
6. In the text field that appears, add 'ctl00$login1$txtUser' as the title.
7. Add your username as the value.
8. Save the item.

This is an example of what the login item would look like: https://share.cleanshot.com/T3V8dfm6

To fill that login, go to the website, click on the 1Password icon in your browser's toolbar, locate the login, and click Autofill. Alternatively, you can go to the website and use a https://support.1password.com/keyboard-shortcuts/ to fill.

Here's a video of both filling methods: https://share.cleanshot.com/nVgQkwSg

I hope that helps. Let us know if you have any other questions.

ref: dev/core/core#20783