This is not a question related to C#, but HTML. There's heaps of clutter in your YouTube source link. Try this puristic link to activate the auto-play function and then add your other link attributes if needed:
string ss = "<!doctype html>" +
"<html><head></head><body>" +
"<iframe id=\"ytplayer\" type=\"text/html\" width=\"1080\" height=\"700\" src=\"https://www.youtube.com/embed/" + youtubeIDS + "?autoplay=1\">" +
"</body></html>";
webbrowser.NavigatetoString(ss);
Apparently, the rel=0
attribute causes issues for some hence this full version may work (just removed rel=0
):
string ss = "<!doctype html>" +
"<html><head><meta bgcolor=\"black\" name=\"viewport\" content=\"width=1080, initial-scale=1.0, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0\" /><title></title></head><body style=background-color:black;>" +
"<iframe \"background-color:black\" id=\"ytplayer\" type=\"text/html\" width=\"1080\" height=\"700\" src=\"https://www.youtube.com/embed/?autoplay=1&loop=1&modestbranding=1&playlist=" + youtubeIDS + "&playsinline=1&controls=0\" frameborder=\"0\" allowfullscreen>" +
"</body></html>";
webbrowser.NavigatetoString(ss);
If everything fails try this link, they discuss a very similar topic: How to make an embedded Youtube video automatically start playing?
UPDATE: If even that doesn't help eliminate all potential influences that may be of a problem, e. g. create an HTML file locally and add the minimal HTML command, then open this file with a couple of Web browsers to check the YouTube auto-play feature. Example that should work:
<html>
<head>
<title>Minimal YouTube Autoplay Example</title>
</head>
<body>
<iframe width="560" height="315" src="http://www.youtube.com/embed/xV7Ha3VDbzE?autoplay=1" frameborder="0"></iframe>
</body>
</html>
Then go from there. If this doesn't work though you have no programming issue to solve ;-)