Namespace attributes are in xmlns namespace, so you should use
XNamespace.Xmlns
+ attributeName
for declaring namespaces:
XNamespace ns = "http://www.sitemaps.org/schemas/sitemap/0.9";
XNamespace video = "http://www.google.com/schemas/sitemap-video/1.1";
var urlset = new XElement(ns + "urlset",
new XAttribute(XNamespace.Xmlns + "video", video));
Produces
<urlset xmlns:video="http://www.google.com/schemas/sitemap-video/1.1"
xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" />
Complete xml generation will look like:
var urlset = new XElement(ns + "urlset",
new XAttribute(XNamespace.Xmlns + "video", video),
new XElement(ns + "url",
new XElement(ns + "loc", "http:/blabla"),
new XElement(video + "video",
new XElement(video + "player",
new XAttribute("allow_embed", "yes"),
"http:/blabla"))));