We have released an update to the Campaign Monitor Add-On that now supports API Client ID. This allows you to pre-set the client by populating the API Client ID on the settings page of the Campaign Monitor Add-On.
Please note that the primary API Key is still required. The Campaign Monitor API Client ID is not an API Key, it is a client id that is used along with the API key to identity a specific client when interacting with the API.
Details on the announcement can be found here: