Codebook's Password AutoFill integration for iOS 12 allows you log into websites (and other iOS apps!) securely using the login and password credentials you store in Codebook. This eliminates the need to interrupt what you are doing, switch apps to your favorite password manager (Codebook, duh), login, lookup the record, copy the password, switch apps back to the login form in question, and paste it. Once you enable Codebook to AutoFill Passwords, iOS will immediately start offering to look up your passwords in Codebook and offer appropriate credentials automatically.
That's it! When the iOS keyboard is activated on a login form, iOS will offer context-specific credentials to choose from, or you can bring up the Codebook AutoFill interface to choose the login you need.
In iOS 12, when you enable Codebook to AutoFill passwords and then visit a login form on the web (or in a properly configured iOS apps), the keyboard will attempt to suggest an appropriate set of credentials from Codebook. If there is no suggestion or you wish to use different credentials, there is a new key button above the keyboard giving you direct access to all your records.
When you tap on the key button at a login prompt and select Codebook from the list you'll be asked to login to Codebook (or to authenticate via Touch ID or Face ID if you have this enabled in Codebook). Once authenticated you'll be presented with a list of potentially relevant records based on the current URL, grouped by how they match. You can choose from the list, or search for the credentials you require.