Please download bug fixes regularly by going to System -> Updates, then click Install Update (For your current version). Alternatively, activate Automatic Bug Fixes in System Settings.
Make sure you have set up Cron in your control panel to run at least once a day. This is very important and required for automation.
Plesk has clear documentation, but the process is a bit too technical for most people: https://docs.plesk.com/en-US/obsidian/api-rpc/about-rest-api.79359/
To add a Plesk server in Hosting Billing you can use either the API token or your password.