On a computer, go to: https://wa.me/{phone_number}
Replace {phone_number}
with the phone number to send to, including the country code and no special characters.
Example: https://wa.me/18001230000
This will open a WhatsApp Web chat to that specific number.
To attach message just append the link with ?text=
Example:
https://wa.me/18001230000?text=Hello%20World!