The API in question doesn't take any input. The bit with { INPUT DATA HERE } is simply part of the template for our code samples and does not indicate any need for input data to be provided for the call. The simplest way to determine if a call requires any JSON input, look for the Post Inputs box on the page for the endpoint. IF that box isn't present, then the endpoint doesn't require a message body.
In the case of this API, the domain is determined either by the provided domain administrator access token, or by the X-SmarterMailDomain header when paired with a system administrator access token.
As for identifying which methods are allowed for a given endpoint, I'm not sure what you mean. The list of methods in each section of the documentation indicates the HTTP method next to each endpoint. In addition, the page for each endpoint has a box labelled Calling the Function that includes both the HTTP method and the relative URL for that endpoint.
Andrew Barker
Lead Software Developer
SmarterTools Inc.
www.smartertools.com