Example
string urlWebsite = "http://MyServer/sites/MySiteCollection";
ClientContext clientContext = new ClientContext(urlWebsite);
Web oWebsite = clientContext.Web;
List oList = oWebsite.Lists.GetByTitle("My List");
UserCustomActionCollection collUserCustomAction = oList.UserCustomActions;
UserCustomAction oUserCustomAction = collUserCustomAction.Add();
oUserCustomAction.Location = "EditControlBlock";
oUserCustomAction.Sequence = 100;
oUserCustomAction.Title = "My First User Custom Action";
oUserCustomAction.Url = urlWebsite + @"/_layouts/MyPage.aspx";
oUserCustomAction.Update();
clientContext.Load(oList,
list => list.UserCustomActions);
clientContext.ExecuteQuery();
