The documentation for adding a Message Filter Extension states that the Associated Domains capability should be added to the Xcode project.
But what does that mean, should it be added to the app extension, to the app, or to both the app extension and the app? The documentation doesn't say which exactly.
You would declare associated domains support (Universal Links) in your extension target. The info.plist to update as described in the documentation is the plist for your app extension as well.
Thank you, however in this answer which I just found
https://forums.developer.apple.com/forums/thread/767859
it says "Your main app should have messagefilter associated domains"
So contradictory information?