The feature in Codebook for macOS and Codebook for Windows that assists in accessing your login details and passwords securely from other apps is called Secret Agent. We’ve recently updated the documentation on how this works and included multiple demonstration videos showing how one can insert a value, fill out a login form, or create a custom form-filling action. If you’re trying to fill out forms with Codebook please have a look, and let us know if you have any feedback.