To use the ID locator to test the username textbox, you should inspect the webpage and see the ID for the particular element.
Like this:
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Now that you know what is the ID for the username's text box, use the following lines of code to test/ provide an input to the text box
driver.get("https://www.facebook.com");
driver.findElement(By.id("email")).sendKeys("username@gmail.com");
This way you can provide an input in the username's textbox.
Hope this helps.