how to send text to textarea in div element using Selenium

Try this:

driver.findElement(By.xpath("//div[@class='input_area']//textarea"))
      .sendKeys("Your Value");

I believe you are mistaken on a few things:

1. You don't send text to the div, you send it to the textarea.
2. The textarea does not have a style of hidden*. It is quite visible in the screenshot you provided.
3. The parent div does not have a style of hidden either*. If it did, you couldn't see the child textarea.
4. The parent div does have style="overflow: hidden" which only hides the content that cannot be seen within it's 'box', but it does not hide the whole element.

I hope this helps. I recommend brushing up on HTML and CSS before going farther in trying to automate the page.

*There could be an external CSS file that could apply the style of hidden, but since we can see the textarea in the screenshot, I don't believe this is the case.

It is possible that the textarea the parent elements above it located inside a frame. Please do check the HTML code of your page and look for any frames wrapping the textarea.

If so, you can temporarily switch to the frame before doing sendKeys():

driver.switchTo().frame(arg). You can choose from any of the signatures below.

/**
 * Select a frame by its (zero-based) index. Selecting a frame by index is equivalent to the
 * JS expression window.frames[index] where "window" is the DOM window represented by the
 * current context. Once the frame has been selected, all subsequent calls on the WebDriver
 * interface are made to that frame.
 *
 * @param index (zero-based) index
 * @return This driver focused on the given frame
 * @throws NoSuchFrameException If the frame cannot be found
 */
WebDriver frame(int index);

/**
 * Select a frame by its name or ID. Frames located by matching name attributes are always given
 * precedence over those matched by ID.
 *
 * @param nameOrId the name of the frame window, the id of the <frame> or <iframe>
 *        element, or the (zero-based) index
 * @return This driver focused on the given frame
 * @throws NoSuchFrameException If the frame cannot be found
 */
WebDriver frame(String nameOrId);

/**
 * Select a frame using its previously located {@link WebElement}.
 *
 * @param frameElement The frame element to switch to.
 * @return This driver focused on the given frame.
 * @throws NoSuchFrameException If the given element is neither an IFRAME nor a FRAME element.
 * @throws StaleElementReferenceException If the WebElement has gone stale.
 * @see WebDriver#findElement(By)
 */
WebDriver frame(WebElement frameElement);