You need to tell us the HTML page, but with a password text field:
<input type="password" id="passwordTextBox">
I would find it using Selenium WebDriver, for example:
IWebDriver firefoxDriver = new FirefoxDriver();
IWebElement passwordTextBox = firefoxDriver.FindElement(By.Id("passwordTextBox"));
Then I will write into it like this:
passwordTextBox.Clear();
passwordTextBox.SendKeys("password");
I would look at the Selenium Web Driver documentation and ask any questions after you read all of this:
http://seleniumhq.org/docs/03_webdriver.html