All the documentation says to hit + Add Product within an existing app and choose instant games but it is not listed.

Examples:

• https://developers.facebook.com/docs/games/build/crossplay/get-started > Using same Application ID on developer.facebook.com, add Instant Game product to it.
• https://developers.facebook.com/docs/games/build/instant-games/get-started > To enable Instant Games for the app, click the + Add Product button at the bottom of the navigation menu.

Same issue reported from 6 months ago with no traction: https://developers.facebook.com/community/threads/694117442564394/