Why you worry, just use activity starter
Set Uri to join ( https://wa.me/)(whatsappphonenumber)(?text=)(urlencodedtext)
Set action to android.intent.action.VIEW
If what’s number is valid, then it will landing at WhatsApp message to that particular and you need to press Send or else it will throw an error , no what’s up found…
(It will work fine though you didn’t have that number in your contact list)
Just sample