How to write the Mat/Image to the RTMP stream in OpenCV - c++

Question

I can open RTSP video stream by using the VideoCapture and read the frames.
It is the URL of RTSP stream:
rtsp://username:password#xxx.xxx.xxx.xxx:554/Streaming/Channels/102
Now I want to send/write the image/mat back to the output stream (RTMP stream over LAN network)
I have already set up the RTMP server which is NGINX, and for a test, I downloaded the FFMPEG, and when running the following command (in CMD) then it works well, and successfully read and write the stream to NGINX server.
ffmpeg -i rtsp://username:password#xxx.xxx.xxx.xxx:554/Streaming/Channels/102 -vcodec copy -acodec copy -f flv rtmp://xxx.xxx.xxx.xxx:1395/mylive/test
And now if I put this rtmp://xxx.xxx.xxx.xxx:1395/mylive/test URL to the video tag of HTML, then the video can be opened.
So the question is how I can push the image through RTMP (NGINX) from my code after processing?
Any suggestion. Thanks in advance!!
Edit:
I used VideoWriter class as the following ways, but no one was working:
VideoWriter writer = new VideoWriter();
writer.open("rtmp://xxx.xxx.xxx.xxx:1395/mylive/test", CAP_FFMPEG, 0, 25,
new Size(640, 480));
//writer.open("ffmpeg -vcodec copy -acodec copy -f flv
//rtmp://xxx.xxx.xxx.xxx:1395/mylive/test", CAP_FFMPEG, 0, 25, new Size(640,
//480));
// or without CAP_FFMPEG
if (!writer.isOpened()) {
System.out.println("open error");
}