Fetch video URL:
$ youtube-dl --get-url https://www.youtube.com/watch?v=BRnPidrKto4
API docs: http://dream.reichholf.net/e2web/#mediaplayercurrent
Play video:
http://192.168.3.42/web/mediaplayerplay?file=4097:0:1:0:0:0:0:0:0:0:http%253a%2F%2Fcweiske.de%2Ftagebuch%2Fimages%2Fkdenlive%2Fkdenlive-lower-third.webm
With the URL, the : of the http:// is double encoded.
Stop current video and end dreambox mediaplayer:
$ curl 'http://192.168.3.42/web/mediaplayercmd?command=exit'