This sample code is a simple server written using JavaScript and Node.js. It demonstrates how to generate a signature for subscription offers. The sample demonstrates:Receiving a request.Generating a cryptographic signature using your private key.Sending back a response with the signature.All of the work is done in routes/index.js. You set up environment variables for your key ID and your private key in the start-server file.
Generating a Subscription Offer Signature on the Server | Apple Developer Documentation