How to type some text in hidden field in WebDriver

First of all you have to change the value of type attribute as text from hidden. The following code using javascript would work for that:

EX:

jse.executeScript("document.getElementsByName('body')[0].setAttribute('type', 'text');");

Now, you are able to type on that text by using WebDriver. So, the overall code for typing with WebDriver using Java and Javascript as follows:

EX:

JavascriptExecutor jse = (JavascriptExecutor)driver;

jse.executeScript("document.getElementsByName('body')[0].setAttribute('type', 'text');");

driver.findElement(By.xpath("//input[@name='body']")).clear();

driver.findElement(By.xpath("//input[@name='body']")).sendKeys("Ripon: body text");