I just want to ask is there any possibility right now to Send Firebase In App Message without using Firebase Console? I want to send In App Message (FIAM) to Android App from .NET C# MVC Web Application. I managed to send Push Notification(FCM) succesfully but what about In App Messaging? Is there any API available for that?
There currently is no API to configure the messages delivered/displayed by Firebase In-app Messaging. If this is a use-case you'd like to see added, I recommend you file a feature request.
The closest work-around at the moment would be to use Firebase Remote Config (which does have an API for setting configuration data) to deliver some value to your clients, and then have your clients display the message on those conditions. This is somewhat similar to what Firebase's In-app Messaging does behind the scenes, although (as far as I know) it uses its own infrastructure to deliver the configuration data.