As per your example URL, please try the following code in order to switch to the iframe for the jHTMLArea and manipulate the body text. The code is relatively straight forward, prepping the driver, navigate to the page, switching to the iframe and then manipulating the body, asserting the change and quitting the driver instance.
@Test
public void jarea() {
String test = "Hello";
System.setProperty("webdriver.chrome.driver", "C:/Automation/Chromedriver.exe");
WebDriver d = new ChromeDriver();
d.navigate().to("http://pietschsoft.com/demo/jhtmlarea/");
WebElement iframe = d.findElement(By.xpath("//iframe"));
d.switchTo().frame(iframe);
WebElement jAreaBody = d.findElement(By.xpath("/html/body"));
jAreaBody.clear();
jAreaBody.sendKeys(test);
assertEquals(jAreaBody.getText(), test);
d.quit();
}