K2 SmartObject OData API request body JSON format for ProcessInstance Start Method?

As a little background, I'm looking to use the SmartObject OData API in K2 Cloud to do some processing of workflow instances. I have managed to do some basic stuff like authenticating and listing workflows and removing single items but I need to do some more complex tasks that use JSON headers with structures instead of single variables (see screenshots).

Is there anywhere I can get the format of the JSON that the system smartobject is expecting when posting a request to the ProcessInstance Start method for example? Or more generally how I find these for other smartobjects?

I can create the json for some of the more basic ones where the parameters are simple single values like "ProcInstID" e.g:

{

  "ProcInstID": 7072

}

However, some of them seem to want things like a ProcessInstanceInfo structure or DataFields structure which I have no idea of the format.

I'll attach a couple of screenshots for reference.

Any help would be really appreciated.

I'm hoping there is some form of reference to these API interfaces otherwise I'm flying blind!