Answer
Build the URL_ attribute using the External Search URL syntax rather than a plain form reference: 'ifsapf:frmActivity?external_search=ACTIVITY_SEQ%3d' || &NEW:ACTIVITY_SEQ, URL-encoding the equals sign as %3d and concatenating in the actual key value dynamically from the triggering record. This makes the resulting Stream notification link open directly filtered to the specific record referenced in the triggering event, rather than opening the form with no specific record pre-selected.