Sets the placeholder every token shows until the drop is revealed: one name, description, and image, from a media_token issued by POST /api/v2/drops/{slug}/items/media. It saves a placeholder base URI to the draft, so publish afterwards to put it onchain. Running POST /api/v2/drops/{slug}/metadata/ipfs later replaces it with the real metadata.

Saving it on a revealed drop changes only the stored item and leaves the drop revealed. To put the placeholder back over a revealed drop on purpose, send unreveal=true: the placeholder is written to the draft, and the next publish un-reveals the drop.

Only ERC-721 SeaDrop V1 drops have a prereveal item; any other drop type returns 400.