Its a little unclear from your message on what exactly you want here.
Still if i assume that you want to make a POST API call which can receive data from Mendix and then make the API call, then Consume a REST API
But if you want others call your API with some data which you then use in mendix to create objects, then make a Published Service which has a Microflow under it.
Does this help? if not, kindly elaborate please to help better