Use the callback operator to enable passing callback functions to API calls. In general, the callback operator can only be used on identifiers of functions that are defined with a callback function attribute such as <ASYNC_CALLBACK_FUNCTION>.
dclfunc
  function asyncCallback <ASYNC_CALLBACK_FUNCTION>
  begin
    Print("callback");
  end AsyncCallback;
dcltrans
  transaction TMain
    begin
      WebAsyncPreparePush(callback(AsyncCallback));
  end TMain;