A mobile website can be customized to allow users to share a pre-filled message in WhatsApp to a manually chosen contact. As given here it is done using Custom URL Scheme. A
As noted by others, the official documentation is available here: WhatsApp.com FAQ: Android -> Chats -> How to use click to chat. The documentation states:
Example: https://wa.me/15551234567?text=I'm%20interested%20in%20your%20car%20for%20sale
BUT! Why don't we try copying that into a new tab in your browser and going there right now?
https://wa.me/text=testtesttesttest
Results: ERROR PAGE NOT FOUND.

What gives???
Fix it easily by using one of THESE format:
https://api.whatsapp.com/send?text=YourShareTextHere
https://api.whatsapp.com/send?text=YourShareTextHere&phone=123
No wa.me domain in this URL!