All of the CTAPI web service calls require a set of values to be sent via the SoapHeader.   Here is how you can send them in your JSON post.

Just add it at the beginning of your JSON payload data.
{
 "mySLHeader":
 {
  "cpnyID": "SLCPNYID",
  "licenseExpiration": "LICENSEEXPIRATION",
  "licenseKey": "LICENSEKEY",
  "licenseName": "LICENSENAME",
  "siteID": "SITEID",
  "siteKey": "SITEKEY",
  "softwareName": "CTAPI",
  "userName": "optionalSLUsername";
 },

"param1": "param1Value",
"param2": "param2Value"
}