How to add a watermark to the saved video with CameraX api in saving time

I am using the CameraX api approach to record the videos and saving them as explained in the documents: https://developer.android.com/training/camerax/video-capture
I tried with ffmpeg-android-java but clearly it needs to process the saved video not adding to the saved frames in real time.

Now, I want to add a watermark to all saved videos in easy and not costly way and without the need to reprocess the video?
Also, if there is no way, what the best and fastest approach to process the video to only add a watermark?