Sorry, I don't think you can do this. Admob is integrated to iOS via firebase. So you would not have access to firebase inside of your .fmp12 filemaker file, which is running inside of the FileMaker SDK "shell".
You can't really change that FileMaker "shell". So things like firebase integration would either have to be done via a plugin written to work with the FileMaker SDK, or a full on native app. Out of the plugin developers I know, I don't know of any of them that are trying to offer monetization by an ad content network.
Thank you for your information. Hope there is a way in the future to solve this problem.