This plugin allows you to show Firebase In-App Messages inside your app. Only works on iOS right now and is in beta.
-- initialize Firebase In-App Messaging
firebaseInAppMessaging.setUserId(id)-- string set id
Make sure you download and include GoogleServices-Info.plist and put in root folder of corona project. Also add your google-services.json to your root folder and useGoogleServicesJson = true in build.settings. Also add google_app_id(App ID) string to android section (see example below)