I want to install a 3rd party SDK that tracks app install when an app launches for the first time, but there is no AppDelegate.swift for extensions. The only possible solution I can think of is actually having an actually standalone app + iMessage Extension. Is there an alternative way to do this with iMessage Extension only?
No. Since an iMessage "app" is technically just an extension, there's no UIApplication class available. I don't think the workaround of having a standalone app would work since that wouldn't be launched when the iMessage app extension is loaded. If you're trying to track the activity of your iMessage app, then you'd need to do that from within your iMessage app, not a standalone app.