DesiredCapabilities capabilities = DesiredCapabilities.internetExplorer();
RemoteWebDriver driver = new
        RemoteWebDriver(new URL("http://localhost:4444/wd/hub"), capabilities);

driver.get("https://<url>");

WebElement userName = driver.findElement(By.name("usr_name"));
userName.sendKeys("ABCD");

WebElement password = driver.findElement(By.name("usr_password"));
password.sendKeys("password");

WebElement login = driver.findElement(By.name("OK"));
login.click();


WebElement popup= driver.findElement(By.name("popup"));
popup.click();

Thread.sleep(1000);

Set<String> windowHandles = driver.getWindowHandles();
System.out.println(windowHandles);