Error creating an invoice via JSON

Error creating an invoice via JSON

Hi all!

I'm sending this request to create an invoice with JSON:

$url = "https://invoice.zoho.com/api/v3/".$this->service."?authtoken=".$this->authtoken."&organization_id=".$this->organization_id."&JSONString=".$this->json_data;
   $curl = curl_init($url);
   curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);//Set to return data to string ($response)
   curl_setopt($curl, CURLOPT_POST, TRUE);//Regular post
   curl_setopt($curl, CURLOPT_HTTPHEADER, array("Content-Type: multipart/form-data") );
   //curl_setopt($curl, CURLOPT_POSTFIELDS, "JSONString=".$this->json_data);

For the last line I tried also: curl_setopt($curl, CURLOPT_POSTFIELDS, $this->json_data);

But I always get this message: {"code":2,"message":"The request passed is not valid."}

Have any idea?

Thank you