Activating camunda start with payload in spring webclient

i a working on camunda run time standalone with external tasks and i want to call the start flow rest in my spring boot application with with a payload that i can get the payload in the variables - i am trying do it with spring webclient but with no success
the following just for the test
//build payload
JSONObject camundaValueJson = new JSONObject();
JSONObject camundaParameterJson = new JSONObject();
JSONObject camundaRequest = new JSONObject();
String s = camundaRequest.toString();

//call start“/process-definition/key/{key}/start”,“PaymentProcessTestId”) .bodyValue(s)

but i get an error no 415 UNSUPPORTED_MEDIA_TYPE
when i am trying to pass a josn object“/process-definition/key/{key}/start”,“PaymentProcessTestId”) …body(BodyInserters.fromValue(camundaRequest))

It calls the camunda external task but i dont get the payload in the variable list of the externalTask

what is the correct way of sending it in spring boot webClinet