Use https://wa.me/<number> where the <number> is a full phone number in international format. Omit any zeroes, brackets, or dashes when adding the phone number in international format.
Examples:
Use: https://wa.me/1XXXXXXXXXX
Don’t use: https://wa.me/+001-(XXX)XXXXXXX
Create your own link with a pre-filled message
The pre-filled message will automatically appear in the text field of a chat. Use https://wa.me/whatsappphonenumber?text=urlencodedtext where whatsappphonenumber is a full phone number in international format and urlencodedtext is the URL-encoded pre-filled message.