As an alternative, you can add a custom attribute named App_Key to your source configuration inside the connectorAttributes object, either through the VS Code plugin or by using the API. :
Then, in your authentication configuration, reference it as follows:
App_Key = $application.App_Key$
First, test with a hardcoded value to confirm that the header works correctly. Since App_Key is sensitive, avoid storing it in plain text. You can configure the attribute as encrypted using the key encrypted, for example:
"encrypted": "App_Key"
A few seconds after saving, the App_Key value will be automatically encrypted.