Unlike messenger, WhatsApp test is “keyword” based. If you proceed with the URL, it will redirect you to WhatsApp. The “keyword” will be automatically populated on your WhatsApp's “input”. Simply send that “characters”. Once you send it, your WhatsApp flow will start.
Please watch this video to know more.
You may need to install WhatsApp for your Laptop/PC or send the URL to your phone so it opens in WhatsApp.