Newbie : Schema for a simple hello operation in Portal

[#1]

I have created a simple Proxy API where a call to v0/hello returns "Hello World" as a string (It calls the apigee demo service url)

(There are no params for the API call)

I then published it in the Portal, using a simple schema (snapshot).

Now I want to invoke the API from the portal, but there does not seem to be a "Test It" or "Execute" button in the portal.

If I add a parameter to the Swagger Schema I see a textbox in the portal, where I can enter a value and press Enter, which I believe does cause the API to be called.

Thanks