Welcome to the RingCentral Community

Thank you, I will give that try :)

That did it, thank you so much

The tutorial was created using the old .Net SDK.

Using the current SDK, please modify the code as follows

var parameters = new CreateMMSMessage();
parameters.from = new MessageStoreCallerInfoRequest { phoneNumber = fromNumber };
parameters.to = new MessageStoreCallerInfoRequest[] { new MessageStoreCallerInfoRequest { phoneNumber = RECIPIENT } };
parameters.text = "Hello World from C#";
var attachment = new Attachment { fileName = "test.jpg", contentType = "image/jpeg", bytes = File.ReadAllBytes("test.jpg") };
parameters.attachments = new Attachment[] { attachment };
var resp = await rc.Restapi().Account().Extension().Mms().Post(parameters);