Symptoms
"If a Workflow activity is specified" condition isn't met when using 'SN' parameter in a Form.
Diagnoses
There are two naming conventions for passing a Process serial number to a Form, SerialNo and SN, which come from Designer and K2 Studio respectively. Using 'SN' will requires an 'Open a Worklist Item' action to fire before the "If a Workflow activity is specified" condition can be used, while 'SerialNo' does not.
Resolution
Call the 'Open a Worklist Item' action in your Form Initializing rule. Any subsequent use of the "If a Workflow activity is specified" condition will be successful.
Alternatively, you can manually build the Task Item URL with the following structure:
".../FormName/?SerialNo=[ProcessID]_[ActivityInstanceDestID]"
using 'Process Instance -> ID' and 'Activity Destination Instance -> ID' as its fields.