The easiest way to get Mobile User ID is to build a hidden screen in your app, which reports the data, which can then be used internally or by your app users to report their MUID, as needed.

For helpful tips on setting up a secret screen with SDK registration information, see this article: https://developer.goacoustic.com/acoustic-campaign/docs/add-a-secret-trigger-to-display-debug-information

For general details about the SDK APIs used to access registration details, see the following article: https://developer.goacoustic.com/acoustic-campaign/docs/detect-the-sdk-on-your-app

If you do not have a hidden screen enabled, it is still possible to get the Mobile User Id by enabling verbose logging on mobile SDK.