Are you trying to call the REST API from a browser and get the result of the REST call to Show Page?
If you are, then the JavaScript that sets up the page won’t have been loaded so Show Page won’t work. The way I’ve worked around this before is to get the API to persist the incoming data to an entity, and generate a deeplink. I then have API call issue a redirect as the result of the call. The browser will then redirect to the deeplink, which loads the Mendix framework and you can retrieve the originally passed data from the persisted entity.
Hope this helps.