What is a custom shared URL?
By default, the link of a story is something like this: https://snkt.io/620e5449e918f8196592cb46. With this option, you can create a customized one like https://snkt.io/panic2instant for instance.
Is the custom shared link available at the story level or is it for the entire account?
It is at the level of the story.
⚠️ It is different from CNAME (custom domain). See more help here.