A free open source utility. Code is at Html Utils
Convert a phone number to a link that you can tap, and whatsapp (if installed on that device or web.whatsapp.com if logged in) will start a message to that number.
Optional add a message to type for initial message.
This page supports two get params - n for number and t for text. Example : https://sel2in.com/pages/prog/tapToMsgMakeUrl.html?n=917000&t=hi+there