Example
string urlWebsite = "http://MyServer/sites/MySiteCollection";
ClientContext clientContext = new ClientContext(urlWebsite);
Web oWebsite = clientContext.Web;
UserCustomActionCollection collUserCustomAction = oWebsite.UserCustomActions;
UserCustomAction oUserCustomAction = collUserCustomAction.Add();
oUserCustomAction.Location = "Microsoft.SharePoint.StandardMenu";
oUserCustomAction.Group = "SiteActions";
oUserCustomAction.Sequence = 101;
oUserCustomAction.Title = "Website User Custom Action";
oUserCustomAction.Description = "This description appears on the Site Actions menu.";
oUserCustomAction.Url = urlWebsite + @"/_layouts/MyPage.aspx";
oUserCustomAction.Update();
clientContext.Load(oWebsite,
webSite => webSite.UserCustomActions);
clientContext.ExecuteQuery();
