input: [0] post-url [1] string [2] 
output: Response content is obtained as result of sending a HTTP POST request to the provided URL. The POST message is always submitted using the multipart/form-data encoding (such as that used by a web browser when an HTML form has the enctype=multipart/form-data parameter set). If http-post-parameters field is given (see below), then the relevant parameters are sent as part of the request. X-Mbuni headers are sent as well. See below for an explanation on how the response content is interpreted.