Where can I preorder a Crown Zenith Booster Box?

The short answer: You can't.  They will only make Elite Trainer Boxes and gift boxes.

Some more info:

Crown Zenith is considered a 'special set' or 'side set.'  Normal sets like Silver Tempest or Brilliant Stars have full booster boxes and packs available for retail sale, but special sets are only available inside gift-type boxes or tins.  The special set packs don't even have barcodes on them!

How can I tell a 'special Pokémon TCG set' from a normal set?

Normal sets will say the name of the 'block' or 'series' before the name of the set.  For instance: Sword and Shield Silver Tempest, Sword and Shield: Lost Origin, XY: Evolutions, XY: Furious Fists, Sun & Moon: Cosmic Eclipse.  Special sets don't say the name of the video game before the name of the set.  Examples of special sets include: Celebrations, Shining Legends, Shining Fates, Hidden Fates, Pokémon GO, etc.