Remember that you will need to make an external mechanism to hanlde the result of async calls. Inside the WorkItemHandler you can get the workItem Id. Two (of the probably infinite) options that you have are:
- Pass the id (along with the session Id) to the external system. The external system, when finishes, can use those values to perform a call to your system and complete the work item.
- Create a complete isolated mechanism for handling async calls to external systems. In this scenario, you could call the external system on your WorkItem Handler and then notify this third system (the isolated system I have mentioned before) that a new "task" is running. When the external system completes its work, it notifies this to the same third system. When a task is completed, the third system will notify your system about it.
Best Regards,