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

Hey @franklaw, 1Password does have the ability to save and fill non-login fields. In some cases, 1Password will save these details automatically. In some cases, you will have to create a custom field in order to add these details to your login.

Please note that while it is possible to create a custom field in the item to hold your info, we can't guarantee that it will fill. The success of filling the custom field is largely dependent on how the form in question is coded. The example you gave unfortunately doesn't provide us enough information to give you a specific answer. Do you have a link to a live form on which an issue like this appears? If so, please let us know.

I assume this means the answer is no, then, with respect to 1Password being able to fill arbitrary HTML input fields? It sounds like you need to process each site on a case-by-case basis. That sounds like a lot of work.
An example of a site that runs into problems is this: https://seicashmanagement.mybankingservices.com/
The HTML in question, I believe, is

@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

About the arbitrary value of 'ctl00$login1$txtUser', there is an official documentation about how to create this glob values?