How can I find the field names for input boxes of apps (not websites)

Hi all! Hope you're doing well!

I searched around, but could not find out how to do this:

I would like to fill in the username, password and OTP of Tunnelblick and would like to know how to find out what the names of the input boxes are. 1Password obviously can see them, and I would like to know what I should name the items.

Tunnelblick is just 1 example, it's just a general question about field name detection.

Best regards,
Sander

---

1Password Version: Not Provided
Extension Version: Not Provided
OS Version: Not Provided
Browser: Not Provided