That's a REST service https://docs.mendix.com/howto/integration/publish-rest-service
I see, so you want
1. A non-persistent entity in your domain model called Request with a String attribute Url
2. A microflow called CreateRequest that creates an empty Request object
3. A microflow GetJson that takes a Request. It has a Call REST Service Action. It does a GET request to the Url in the Request. Put the result in a String. Then have a 'Show message' action that shows the JSON
3. A page that shows a dataview that takes that the CreateRequest microflow as its source
4. A button on the page that calls the GetJson microflow
When I click on the front page, the following page appears: