Add Chrome Extension (Download the CRX file first and provide the path to it)

ChromeExtension_PlugIn_Test.java

import java.io.File;
import java.net.URL;
import org.openqa.selenium.OutputType;
import org.openqa.selenium.TakesScreenshot;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeOptions;
import org.openqa.selenium.remote.DesiredCapabilities;
import org.openqa.selenium.remote.RemoteWebDriver;

public class ChromeExtension_PlugIn_Test {

	public static void main(String[] args) {
		
		String USERNAME = "<BROWSERSTACK_USERNAME>";
		String AUTOMATE_KEY = "<BROWSERSTACK_ACCESS_KEY>";
		String HUB_URL = "https://" + USERNAME + ":" + AUTOMATE_KEY + "@hub-cloud.browserstack.com/wd/hub";

		String AUT_URL = "https://www.whatsmybrowser.org/";
		
		WebDriver webDriver = null;
		ChromeOptions options = null;
		URL URLObj = null;
		DesiredCapabilities caps = null;

		try {

			URLObj = new URL(HUB_URL);

			options = new ChromeOptions();
			options.addExtensions(new File("/Users/test/Downloads/Page Ruler.crx")); // DOWNLOAD THE CRX FILE
			
			caps = new DesiredCapabilities();

			caps.setCapability(ChromeOptions.CAPABILITY, options);
			caps.setCapability("browser", "Chrome");
			caps.setCapability("browser_version", "65.0");
			caps.setCapability("os", "Windows");
			caps.setCapability("os_version", "10");
			
			caps.setCapability("project", "Test Run");
			caps.setCapability("build", "Support Automate");
			caps.setCapability("name", "Test: Add Chrome Extension/PlugIn");
			
			caps.setCapability("acceptSslCerts", "true");
			caps.setCapability("browserstack.debug", "true");
			caps.setCapability("browserstack.console", "verbose");
			caps.setCapability("browserstack.selenium_version", "3.11.0");
			
			webDriver = new RemoteWebDriver(URLObj, caps);
			
			webDriver.manage().window().maximize();
			webDriver.get(AUT_URL);
			Thread.sleep(10000);
			take_screenshot(webDriver);
			
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			if(webDriver != null){
				webDriver.quit();
			}
		}
	} // MAIN END
	
	public static void take_screenshot(WebDriver webDriver){
		try {
			TakesScreenshot scrShot =((TakesScreenshot)webDriver);
			File SrcFile=scrShot.getScreenshotAs(OutputType.FILE);
//			File DestFile=new File("/Users/test.png");
//			FileUtils.copyFile(SrcFile, DestFile);
		} catch (Exception e) {
			 e.printStackTrace();
		}
	} // FUNC END
} // CLASS END