Selenium code to open facebook messenger and send a message

Question

I'm just starting to learn a little bit of Selenium scripting (in Java). Currently I'm trying to open a chat box in Facebook and send a message.
I have gotten up to being able to open the chat box through selenium, however I can't figure out how to actually type things into the text box.
Currently here is my code:

import org.openqa.selenium.*;
import org.openqa.selenium.chrome.ChromeDriver;

public class practice {
    public static void main(String[] args) throws InterruptedException {
      System.setProperty("webdriver.chrome.driver", "C:/max/chromedriver.exe");

      WebDriver driver = new ChromeDriver();
      driver.get("http://facebook.com/login/");
      driver.manage().window().maximize();
      driver.findElement(By.name("email")).sendKeys("[email protected]");
      driver.findElement(By.name("pass")).sendKeys("myPassword");
      driver.findElement(By.id("loginbutton")).click();
      driver.findElement(By.name("mercurymessages")).click();
      driver.findElement(By.cssSelector("a[href*='https://www.facebook.com/messages/conversation-8148306']")).click();
     // This has worked randomly. Sometimes the driver will work and open the chat box. Sometimes it will say element not found. I didn't include the full link of the conversation because apparently you don't have to. And it has worked like this in the past.
}
}

My current issue I'd like resolved is: why does this only work sometimes, and how do i find the text box area on the chat box? I did an Inspect Element and the chat box is very odd. It doesn't have anything like an id or a name, so i can't do By.id, or By.name. I can't figure out how to do it with By.cssSelector. This is what the inspect element for the text box looks like:

textarea class="uiTextareaAutogrow _552m" data-ft=".... more stuff here" onkeydown=" more stuff here" style= "more stuff here."

Solution

You have to learn Xpath and how to create Relative xpath.The xpath for the textarea

driver.findElement(By.xpath("//textarea[@class='uiTextareaAutogrow _552m']"));

Anyhow I've made few changes that Include instead of clicking on some other message.It will create a new message and send to your friend

driver.findElement(By.name("mercurymessages")).click();

    //wait for 20 seconds 
    WebDriverWait wait = new WebDriverWait(driver, 20);
    wait.until(ExpectedConditions.visibilityOfElementLocated(By.id("u_0_5")));
    driver.findElement(By.id("u_0_5")).click();//To click on Send a New Message Link

    //To enter a name into the to field 
    wait.until(ExpectedConditions.visibilityOfElementLocated(By.xpath("//input[@class='inputtext textInput']")));
    WebElement friendName = driver.findElement(By.xpath("//input[@class='inputtext textInput']"));

    friendName.sendKeys("Deep");//Change it with your friend name

    //wait for the user list to appear
    wait.until(ExpectedConditions.presenceOfElementLocated(By.xpath("//li[@class='user selected']")));
    friendName.sendKeys(Keys.ENTER);

    WebElement messageBox = driver.findElement(By.xpath("//textarea[@class='uiTextareaAutogrow _552m']"));
    wait.until(ExpectedConditions.visibilityOf(messageBox));
    messageBox.sendKeys("Hi there");

    messageBox.sendKeys(Keys.ENTER);