Hey!
Indeed, you can’t directly use @forge/api in a CustomUI application, you have to define a Forge function first with @forge/resolver then invoke this function in your Custom UI app via the @forge/bridge package.
You can also use the requestJira or requestConfluence functions from the same package, but note that will call the product API on the current logged user behalf.
See @forge/resolver documentation and forge/bridge documentation.
If i develop a forge application using UI kit, can i publish it in atlassian marketplace?
Using Custom UI or UI Kit doesn’t impact the publication on the marketplace.