CefSharp 3 set proxy at Runtime

thanks to amaitland the proper way to actively inforce changing the request-context prefrences, is to run the code on CEF UIThread as following:

    Cef.UIThreadTaskFactory.StartNew(delegate {
        var rc = this.browser.GetBrowser().GetHost().RequestContext;
        var v = new Dictionary<string, object>();
        v["mode"] = "fixed_servers";
        v["server"] = "scheme://host:port";
        string error;
        bool success = rc.SetPreference("proxy", v, out error);
        //success=true,error=""
    });

if anyone need any other soulition i found this solution.

Cef.UIThreadTaskFactory.StartNew(delegate
        {
            string ip = "ip or adress";
            string port = "port";
            var rc = this.browser.GetBrowser().GetHost().RequestContext;
            var dict = new Dictionary<string, object>();
            dict.Add("mode", "fixed_servers");
            dict.Add("server", "" + ip + ":" + port + "");
            string error;
            bool success = rc.SetPreference("proxy", dict, out error);

        });

I downloaded CefSharp.WinForms 65.0.0 and made class, that can help to start working with proxy:

public class ChromeTest
{
    public static ChromiumWebBrowser Create(WebProxy proxy = null, Action<ChromiumWebBrowser> onInited = null)
    {
       var result = default(ChromiumWebBrowser);
        var settings = new CefSettings();
        result = new ChromiumWebBrowser("about:blank");
        if (proxy != null)
            result.RequestHandler = new _requestHandler(proxy?.Credentials as NetworkCredential);

        result.IsBrowserInitializedChanged += (s, e) =>
        {
            if (!e.IsBrowserInitialized)
                return;

            var br = (ChromiumWebBrowser)s;
            if (proxy != null)
            {
                var v = new Dictionary<string, object>
                {
                    ["mode"] = "fixed_servers",
                    ["server"] = $"{proxy.Address.Scheme}://{proxy.Address.Host}:{proxy.Address.Port}"
                };
                if (!br.GetBrowser().GetHost().RequestContext.SetPreference("proxy", v, out string error))
                    MessageBox.Show(error);
            }

            onInited?.Invoke(br);
        };

        return result;
    }

    private class _requestHandler : DefaultRequestHandler
    {
        private NetworkCredential _credential;

        public _requestHandler(NetworkCredential credential = null) : base()
        {
            _credential = credential;
        }

        public override bool GetAuthCredentials(IWebBrowser browserControl, IBrowser browser, IFrame frame, bool isProxy, string host, int port, string realm, string scheme, IAuthCallback callback)
        {
            if (isProxy == true)
            {
                if (_credential == null)
                    throw new NullReferenceException("credential is null");

                callback.Continue(_credential.UserName, _credential.Password);
                return true;
            }

            return false;
        }
    }
}

Using:

        var p = new WebProxy("Scheme://Host:Port", true, new[] { "" }, new NetworkCredential("login", "pass"));
        var p1 = new WebProxy("Scheme://Host:Port", true, new[] { "" }, new NetworkCredential("login", "pass"));
        var p2 = new WebProxy("Scheme://Host:Port", true, new[] { "" }, new NetworkCredential("login", "pass"));

        wb1 = ChromeTest.Create(p1, b => b.Load("http://speed-tester.info/check_ip.php"));
        groupBox1.Controls.Add(wb1);
        wb1.Dock = DockStyle.Fill;

        wb2 = ChromeTest.Create(p2, b => b.Load("http://speed-tester.info/check_ip.php"));
        groupBox2.Controls.Add(wb2);
        wb2.Dock = DockStyle.Fill;

        wb3 = ChromeTest.Create(p, b => b.Load("http://speed-tester.info/check_ip.php"));
        groupBox3.Controls.Add(wb3);
        wb3.Dock = DockStyle.Fill;