I am wondering if it's possible to share a youtubevideo on facebook and make it start automatically in the timeline. (I have also the possibility to share a landingpage with the youtube iframe and open graph metatags, but i could not find anything about autoplay.)
Or is the autoplay function only available on videos upload directly to facebook?
you can't control the autoplay feature on your side by adding autoplay=1 or somethingelse to your video url. It depends on the user, having autoplay for videos in his feed activated or not. You can check this on your settings page --> https://www.facebook.com/settings?tab=videos
There you can find the autoplay option. Furthermore autoplay is only available for videos on facebook and not for youtube, vimeo, etc. videos.