Create a new folder on any driver except C: Drive. The folder name should be “Chrome_Test_Profile”.

Set the chrome application path to an environment variable. Follow the below steps to set environment path in windows:

In Search, search this: System (Control Panel)

Click the Advanced system settings link.

Click Environment Variables. In the section System Variables, find the PATH environment variable and select it. Click Edit. If the PATH environment variable does not exist, click New.

In the Edit System Variable (or New System Variable) window, append the path. (E.g. C:\Program Files (x86)\Google\Chrome\Application).  

Once you are done with all the above, setup follow the below steps:

1. Open Chrome in debug mode using the following command.

chrome.exe -remote-debugging-port=9014 --user-data-dir="E:\Selenium Scripts\Chrome_Test_Profile” (This command opens the window in debug mode).

You can use any port, here I am using 9014.

By executing the above command, it will create a new profile for the chrome. It will not affect the original profile of chrome.

2. Now open https://web.whatsapp.com  and login with your account in chrome. Which is already running in debug mode.

3. Now Open Eclipse and create a new java project.

File > New > Java Project

Write Project Name (e.g. WhatsappMsgSender) and click on next.

Now go to library > Add external Jars ( Add  All selenium Required jar files )Selenium standalone server and other Required jar files you can download from this link: https://www.seleniumhq.org/download/

Click on finish.

4. Now create a new package as below:
1. Right-click on project > new > package
2. Write a package name (e.g. whatsappmsg).
3. Click on finish

5. Now create a new class in the above-created package.
1. Right-click on project > new > Class
2. Write class name (e.g. SendMsg).
3. Click on finish

6. Create one main method in the above-created class and add following code for executing script in chrome which is already open in debugging mode in step 2.

System.setProperty("webdriver.chrome.driver", "E:\\Selenium Scripts\\libs\\chromedriver.exe"); 	
 
ChromeOptions options = new ChromeOptions();
options.setExperimentalOption("debuggerAddress","localhost:9014");
 
WebDriver driver= new ChromeDriver(options);
driver.manage().timeouts().implicitlyWait(60,TimeUnit.SECONDS);

Above code will interact with the already open chrome browser.

7. Write Following code for clicking on new message

Driver.findElement(By.cssSelector("#side > header > div.sbcXq > div > span > div:nth-child(2)")).click();

Thread.sleep(2000);

8. To read contact number and message, create a comma Separated CSV File in below format.

CSV Format:

Name 1, Message 1,

Name 2, Message 2

10. Write the following code to read a contact number and message from a CSV file.

try (BufferedReader br = new BufferedReader(new FileReader(csvFile))) 
 {
 	while ((line = br.readLine()) != null) 
	{
    	ContactInfo = line.split(cvsSplitBy);
   	   	int i = 0;
   	   	int j = 1;
		driver.findElement(By.cssSelector("#app > div > div > div._37f_5 > div._3HZor._3kF8H > span > div > span > div > div:nth-child(2) > div > label > input")).sendKeys(ContactInfo[i]);
		Thread.sleep(2000);
		Actions action = new Actions(driver);
		action.sendKeys(Keys.ENTER).build().perform();
		driver.findElement(By.cssSelector("#main > footer > div._2i7Ej.copyable-area > div._13mgZ > div > div._3u328.copyable-text.selectable-text")).sendKeys(ContactInfo[j]);
		Thread.sleep(2000);
		action.sendKeys(Keys.ENTER).build().perform();
		driver.findElement(By.cssSelector("#side > header > div.sbcXq > div > span > div:nth-child(2)")).click();
		Thread.sleep(2000);
        i++;
        j++;	            	
	}
 } 
catch (IOException e) 
{
       e.printStackTrace();
}

Reference:

1. Refer the below link on how to execute Selenium scripts on an already opened browser: -?  
https://youtu.be/4F-laDV9Pl8