When you start process, this execute the first node into process sequence. For retrieve task you need query for Assigned Tasks or Reserved Task or Ready Task
In general, addTask method of class TaskClient accepts implementation of AddTaskResponseHandler which will get task id.
But ... after quick look at WSHumanTaskHandler implementation it does not use response handler at all. So you could modify it if you like to do code changes in jBPM or use one of the query methods provided by TaskClient to get tasks available based on certain search criteria, as already mentioned by Jose.