It can fire after 900 if the Byzantine emperor is not christian or if Thrace or Ancyra are not controlled by christians or if Athens and Corinth both arent controlled by christians. Also if any places out of Rome, Provence, Toulouse, Paris, are held by either non-christians or heretics. Or finally if the counties (or duchies not sure) of Cologne(Köln) or Braunschweig are christian but the ones holding them are non-christian.
Beyond that it needs to be at least 1090 and Jerusalem needs to not be held by christians.
To answer your actual question, if youre trying to trigger the crusades starting with the console commands there is 4 event ids.
1090 AND JERUSALEM NOT CHRISTIAN
narrative_event = { id = 39660
BYZANTIUM FAILING
narrative_event = { id = 39661
ROME HAS FALLEN
narrative_event = { id = 39662
EUROPE
narrative_event = { id = 39663
The part youd want to enter after "event" in the console is the number at the end. Im not sure if it needs to be in < > or not