Connecting external video source
ffmpeg -re -i Avengers2.mp4 -vcodec flv -f flv -rtmp_conn "O:1 NS:securityCode:SID_OF_EXISTENT_USER NN:width:720 NN:height:480 O:0" rtmp://localhost:1935/openmeetings/ROOM_ID/UNIQUE_BROADCAST_ID_STRING
ffmpeg -re -i Avengers2.mp4 -vcodec flv -f flv -rtmp_conn "O:1 NS:securityCode:SID_OF_EXISTENT_USER NN:width:720 NN:height:480 O:0" rtmp://localhost:1935/openmeetings/ROOM_ID/UNIQUE_BROADCAST_ID_STRING