Exchange REST API CORS Issue

I've created an add-in for Outlook which uses the Exchange REST API to load email headers and forward emails. In my O365 environment it works without any problem. Now I try to deploy the add-in at one of my clients.

They have a classic on-premise Exchange 2016 server. The deployment of the plugin works, but now I have problems that the IIS / Exchange blocking my API request because of CORS. The Plugin is hosted on an external server.

I've got the following error: 

I think I have to make some changes to the HTTP Response Headers in the IIS. I tries something like this: