How to add a Sticker App Extension to an existing app with an iMessage Extension?

I have a working iMessage app in the app store that does NOT have a companion app. iMessage app only. I now want to add stickers to it. I cannot get this to work; stickers do not show up in the stickers section, nor in the "Manage Sticker Apps" section.

Problem exists on Xcode 16, latest OS, on all simulators and devices.

• I tried adding a new "Sticker App Extension" target to the project.
• I tried adding just the Stickers.xcassets file to the exsisting iMessage extension and the app itself.
• I tried googling but all examples and tutorials show how to create a complete stickers app, which works correctly. But I do not want the app.

Notable mention:

• If I create a new "Stickers app" project it works, but if I then try to add an iMessage app extension, THAT does not work so I then have a reverse situation in which the stickers extension DOES work but the iMessage app doesn't.

I somehow have the feeling that an app cannot have both an iMessage Extension and a Stickers App Extension but the documentation is, of course, in true Apple style, lacking.

So how to add a Sticker App Extension to an existing app with an iMessage Extension?