I am facing the same issue. Launched the app over a week ago and no results in the search by name, developer name, or even App Store ID. Going to look into if there is any other way to link the app to AdMob but keep this thread updated if you find anything.

UPDATE: It seems that this is recently a very common issue as AdMob has been going through changes. Myself and it seems many others on the AdMob forum, have been able to successfully link the app from the App Store to AdMob just by reaching out. See link below to the forum and just post a new conversation with your issue including App Store link, and someone should be able to help within 24 hours it seems. Good luck! AdMob Developer Forum - https://developer.apple.com/forums/content/attachment/1368829a-89b2-408e-a109-a64c94385fb6

It looks like this is now available on the API with the postEvent(_:) method on GroupSession: https://developer.apple.com/documentation/groupactivities/groupsession/postevent(_:)-1dno2