I found the answer:
PhantomJSOptions options = new PhantomJSOptions();
options.AddAdditionalCapability("phantomjs.page.settings.userAgent", "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:26.0) Gecko/20100101 Firefox/26.0");
IWebDriver driver = new PhantomJSDriver(options);
Thanks.