Instead of directly exposing TMS:Task form.. You can create staging form. Define a filter on TMS:Task on submit with Run If Qual as 'TR.TaskId != $NULL$' AND 'DB. TaskId = $NULL$'. If true then push required details to staging form. You can define web service on staging form and publish the same.
If you do not wish to use staging form then simple define filter on TMS:Task on submit with above qualification.