Basically a common integration towards a REST service, by calling their API.
0 – find their API documentation to know which endpoints to call and how (how do they expect you to send a request, and how to they response)
1 – create a page in mendix
2 – create a domain model that supports your conversations (user-conversation-message ,...), requests, responses and related logic (data sources that makes, associates stuff)
3 – create a message on that page, so you can capture the content, to make a request for the SAP REST service
4 – create a message for the received response, associate it to the same conversation of the sent message