Unable to Post JSON to custom actions: Getting 415 Unsupported Media Types

I am posting JSON as :

{"contractId":[1,3,5,7,9],"projectId":9
}

I have configured custom action as:

var deleteContracts = builder.EntityType<Contract>().Collection.Action("DeleteItems").Returns<HttpResponseMessage>();
deleteContracts.CollectionParameter<long>("C_Id");
deleteContracts.Parameter<long>("P_Id");

I have written my Action as :

[HttpPost]
public IHttpActionResult DeleteItems (ODataActionParameters param)
 {
    //DO something
    return null;
 }

and i am getting this error data:

"error": {"code": "","message": "The request entity's media type 'application/x-www-form-urlencoded' is not supported for this resource.","innererror": {"message": "No MediaTypeFormatter is available to read an object of type 'ODataActionParameters' from content with media type 'application/x-www-form-urlencoded'.","type": "System.Net.Http.UnsupportedMediaTypeException", "stacktrace": " at System.Net.Http.HttpContentExtensions.ReadAsAsync[T](HttpContent content, Type type, IEnumerable`1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken) at System.Web.Http.ModelBinding.FormatterParameterBinding.ReadContentAsync(HttpRequestMessage request, Type type, IEnumerable`1 formatters, IFormatterLogger formatterLogger, CancellationToken cancellationToken)"
}
}

Would please someone help me to understand what am i doing wrong?

Request Header is as:

POST /ContractsWebServices/ContractsWebService/odata-contracts/Contracts/Default.DeleteItems HTTP/1.1
HOST: localhost:65314 content-type: application/json content-length: 50

{ "contractId":[1,3,5,7,9], "projectId":9 }

any help would be appreciated?