You need to open command prompt and use this command :
BrowserStackLocal.exe -proxyHost 2.2.2.2 -proxyPort 8080 -proxyUser xyz -proxyPass sksk -forcelocal xyz
You can use this simple code for learning purpose :
public static final String USERNAME = "xyz";
public static final String AUTOMATE_KEY = "xyz";
public static final String URL = "http://" + USERNAME + ":" + AUTOMATE_KEY + "@hub.browserstack.com/wd/hub";
public static void main(String[] args) throws Exception
{
DesiredCapabilities caps = new DesiredCapabilities();
System.setProperty("java.net.useSystemProxies", "true");
System.setProperty("http.proxyHost","2.2.2.2");
System.setProperty("http.proxyPort","8080");
System.setProperty("http.proxyUser","xyz");
System.setProperty("http.proxyPass","xyz");
caps.setCapability("browser", "FireFox");
caps.setCapability("browser_version", "40.0");
caps.setCapability("os", "Windows");
caps.setCapability("browserstack.debug", "true");
caps.setCapability("browserstack.local", "true");
WebDriver driver = new RemoteWebDriver(new URL(URL), caps);
driver.get("http://www.google.com");
WebElement element = driver.findElement(By.name("q"));
element.sendKeys("BrowserStack");
element.submit();
System.out.println(driver.getTitle());
driver.quit();
}