UserCustomActionCollection class

public sealed class UserCustomActionCollection : ClientObjectCollection<UserCustomAction>

using System;
using Microsoft.SharePoint.Client;

namespace Microsoft.SDK.SharePointFoundation.Samples
{
    class UserCustomActionCollectionExample
    {
        static void Main()
        {
            string siteUrl = "http://MyServer/sites/MySiteCollection";

            ClientContext clientContext = new ClientContext(siteUrl);
            Web site = clientContext.Web;

            UserCustomActionCollection collUCA = site.UserCustomActions;
            UserCustomAction newUCA = collUCA.Add();
            newUCA.Location = "Microsoft.SharePoint.StandardMenu";
            newUCA.Group = "SiteActions";
            newUCA.Sequence = 1000;
            newUCA.Title = "New Menu Item";
            newUCA.ImageUrl = "/_layouts/images/myIcon.jpg";
            newUCA.Description = "Menu item added";
            newUCA.Url = "/_layouts/create.aspx";
            newUCA.Update();

            clientContext.ExecuteQuery();

            Console.WriteLine("New menu item added to Site Actions menu.\n\nTo view the new menu item, refresh the page.");
        }
    }
}