Ok, well, I just went ahead and added both custom fields so it would auto-populate, and it works fine... but, when I added the PIN field that works as the password field as a password field, it didn't populate it... I had to add it as a plain text field to get it to auto-populate.
Why? Why would it work (auto-populate) when the custom field is added as a text field, but not when it is added as a password field?