Using Safari: If you go to web.whatsapp.com, you will notice that you are being re-directed to the homepage of WhatsApp and the WhatsApp Web simply will not load.
At the top right corner of Safari, find and tab on the "Share" icon (square with up arrow).
From the "Share" menu, find and tab on the "Request Desktop Site" option.
Now, Safari loads again the web.whatsapp.com webpage but this time will load the WhatsApp Web instead. Follow instructions to add the device linked to your phone with WhatsApp. Remember, you should have WhatsApp installed on your phone first. Well, that's how WhatsApp is working.
Note that the web version of WhatsApp has some limitations compared to the app version. Also, at the left panel, you will have to tab 2 times to select, instead of one time in the normal WhatsApp app.
For Chrome: If you choose to use Chrome on iPad as your web browser, the way to load WhatsApp web is the same. Just the "Request Desktop Site" is under the 3 vertical dots (...).