As a KeePassXC user, this extension is absolute gold. Getting it to work isn't entirely intuitive. Tips from a happy user:

Make sure the KeePassXC database has permission to link by checking off the relevant boxes under Tools > Settings > Browser Integration and Database Settings > Browser Integration. Don't close KeePassXC. Go to the extension and connect it under Settings > Connected Databases. In order for this extension to function, KeePassXC *must be running*. In KeePassXC, you can choose to minimize or hide the program instead of quitting under Tools > Settings. In the KeePass entry itself, don't forget to specify a URL.