Restrict request for additional Information in Candidate Selection workflow

Case Background:

We request candidate to fill in "candidate Engagement Survey" after applying to a requisition (externally). Which is configured on candidate selection workflow on the state of "to be reviewed".

Requirement:

Our requirement here is to ensure that only those candidates which have applied on the requisition to receive this request.

Currently the candidates which are marked manually over the requisition by recruiters are also receiving this.

Is it possible to restrict the request for information to only those candidates which apply only from the external career site.