You have the ability to send REST messages using Flow Designer. This can be done using the REST Step after enabling Integration HUB. You need to create a REST step and fully define the REST message. In the REST step you define the connection settings where you configure if it uses a MID server. Unlike the normal OOB Rest messages, the flow designer does NOT use a REST probe which contains the payload and destination host.
Usually the destination is sent in the ECC queue payload and the MID server will resolve the hostname within the MID server and send the message. In Flow Designer and Integration HUB this is different. The instance sends a IPaaSActionProbe message instead of a RESTProbe. The MID server communicates directly to the instance to fetch and return the REST message. One other difference is that with Integration HUB, the instance will actually try to resolve the destination Host BEFORE sending the message through the MID server. Unfortunately, most of the time the destination host is not in the public DNS and this REST message will not even go out.
When using the REST spoke in IntegrationHub and using a MID application you receive the following issue:
Unable to resolve IP Address for the host name: xyz.domain.com at com.snc.mid.util.midselector.IPRangeMidFilter:242
when testing your flow.
Create a property on the instance - 'mid.selector.enable_blocking_host_resolution' and set this property to true to bypass the forced host resolution step in the preflight check.