Json post request ( secure) error

Hi  
i want to make a Json Post request in .net it must be secure i havce the .cer file , .pem file .pfx file the private key i wrote some code to that but the server on the other side say the request is missing the private key in it so plz help me in that i want this to be done i have HTTPs on my server i send also the .cer file to the client and they say they installed it
i always gget this error ( bad request 400 bad requets)