I am trying to stream myself playing League of Legends using the XBox app on a Windows 10 PC, I heard it was possible but cannot manage to find the right setting. Has anyone else been there ?
Unfortunately from what I've seen, this feature isn't available yet.
Searching online for it shows some leaked screenshots hinting that it was being worked on, but as of the latest non-insider release (Creators Update), this is currently not in the feature set of Game DVR.
If you really want to stream, and don't mind not using Windows Game DVR, then I recommend you to check out OBS, XSplit or Gameshow. Configuration process of all three for twitch are documented on this page.