You can easily expose a REST service using Mendix. Message Definitions, JSON structures, import and export mappings, all are added the same way you would add another microflow, enum or any other resource. Just right click on the module or folder and look under "add other".
You can also read this blog (https://www.linkedin.com/pulse/how-make-most-out-mendix-native-rest-publish-willem-van-zantvoort/) about publishing REST in Mendix.
All the best,