My party has unknowingly done a large favor for an important member of the Zhentarim by rescuing his daughter. They don't know he is a member of the Zhentarim, and did the rescue for selfless reasons. The Zhentarim wizard is high enough up to know about some previous adventures that the party did for allies of the Zhentarim.
The adventures are set in the 1350s DR, so this is the really evil Zhentarim as noted by Ed Greenwood as @TheEdVerse, not the kind of shady faction of the 1490s DR. The party members are mostly from Shadowdale though they've spent a lot of time in Daggerdale and are currently in Zhentil Keep itself. The party is made up of mostly good aligned characters who have been quietly working to oppose the Zhentarim, while publicly taking some missions from Zhentarim allies.
How would the Zhentarim try to recruit such a party? I am looking for actual recruit attempts or plot hooks from published adventures or books with references. I prefer more of the people are the real monsters feel of adventures like The Doom of Daggerdale than the classic hack and slash of In Search of the Unknown.