带有ContentType Multipart / form-data的WebException

i am trying to make a httpwebrequest, to connect with a api of a customer. This works for most of the apis, but one of them requires ContentType = "multipart/form-data", which why i have to add it to my request. But since i added it, i get an System.Net.WebException at the line where i am using the GetResponse-method. My JsonRequest is correct, because i tried it at a demo site, but i do not know what to do with this Exception! If i do not at the ContentType multipart/form-data, i get an error msg from the api, that "not all parameters are set correctly". I hope you can help me!i am trying to make a httpwebrequest, to connec