What's the correct ATC phraseology to enter a nonpublished hold?
You didn't mention any particular country or jurisdiction, but for the US the FAA's ATC orders say in section 4-6-4:


  PHRASEOLOGY−
   HOLD (direction) OF (fix/waypoint) ON (specified radial, course, bearing, track, airway, azimuth(s), or route.)  
  
  If leg length is specified, 
  (number of minutes/miles) MINUTE/MILE LEG.  
  
  If
  direction of turn is specified, 
  LEFT/RIGHT TURNS.