public static string proxyAddress;
public static int proxyPort;
public static string proxyUsername;
public static string proxyPassword;

ChromeOptions options = new ChromeOptions();
..............sonrasına
string proxyInfo = "ip:port:user:pass";

string[] parts = proxyInfo.Split(':');
if (parts.Length == 4)
{
    proxyAddress = parts[0];
    proxyPort = Int32.Parse(parts[1]);
    proxyUsername = parts[2];
    proxyPassword = parts[3];
    options.AddArgument($"--proxy-server={proxyAddress}:{proxyPort}");
}

IWebDriver driver = new ChromeDriver(service, options);
IWebElement element = null;
................ hemen sonrası
var networkAuthenticationHandler = new NetworkAuthenticationHandler
{
UriMatcher = uri => true,
Credentials = new PasswordCredentials(proxyUsername, proxyPassword)
};
var networkInterceptor = driver.Manage().Network;
networkInterceptor.AddAuthenticationHandler(networkAuthenticationHandler);
await networkInterceptor.StartMonitoring();