I realize you has asked this question just over a year ago, but I just saw the question now.
The Custom HTTP Runner works just like the standard Web Page Runner, except that you can specify an HTTP method (verb) and the HTTP entity body. (i.e., the body of the request)
The HTTP entity body for a POST request will be the application/x-www-form-urlencoded data. For example, if you are going to POST to the URL with some form data:
URL:
http://www.example.com/controller/HandleAction.doForm data:
name=Daniel
date=2013-03-30
note=Lorem ipsum dolor sit amet
Then the relevant info you'd put in the Custom HTTP Runner would be:
HTTP Method: POST
URL:
http://www.example.com/controller/HandleAction.doHTTP Entity Body: name=Daniel&date=2013-03-30¬e=Lorem%20ipsum%20dolor%20sit%20amet
Basically, the HTTP Entity Body looks very similar to a GET request's querystring.
Hopefully this is helpful, if not to you, to anyone else who might be searching the forums.
Daniel James