I am investigating switching from LastPass to 1Password and am having difficulty with websites that have nonstandard login fields. For example the login fields for my local library account are labele...
You could try filling out all the fields you need to log in and then save the credentials with the help of the browser extension. The new entry should have all fields included.
Let me show an example. I got a website where 3 fields have to be filled out. The first field is a custom field.
Now I type in all the information and use the inline 1Password icon to save the entry as a new one.
Now when opening the website you could see the fields again and their defaults. I click into a field and select the formerly saved new entry with its custom field.
As you can see, the custom field Benutzergruppe is filled automatically, too.
So it should work to fill out custom fields. But maybe you have to let 1Password detect the fields and create a new entry from specified website.