Hi Sourabh,
You need to call the other workflow using the following syntax xtk.workflow.PostEvent ("wkf_internalName", name_activity) .
So if you have a signal on the other workflow which you want to call using PostEvent is should be like :xtk.workflow.PostEvent ("wkf_internalName", "signal") .
Then you need to make sure the workflow with signal activity is started and signal being the first activity.
Regards,
Adhiyan