Try write #{msg} in colomn "To Value", if you want set String "msg" to you Service Task.
If you want Get parameters from service task add "results"
[
"name" : "GetConstant",
"parameters" : [
"ReqType" : new StringDataType(),
"Name" : new StringDataType()
],
"results" : [
"Result" : new UndefinedDataType()
],
"category" : "Const",
"displayName" : "GetConstantByName",
"icon" : "..."
],