<a href="https://api.whatsapp.com/send?phone=910000000000&text=Hello this is the starting message">Whatsapp Chat</a>
If you want to add a whatsapp chat button on your website that will open whatsapp application on your mobile if you are browsing website on the mobile. If you are viewing the website on Laptop then it will open the web version of whatsapp application.
You need to pass two parameters to whatsapp api endpoint calling by using href attribute of <a> tag HTML.
When you will click on the link you will see the screen like above if you are opening it on your computer system.