With Tap Tap, you won't need to add every number to your contacts before talking to them. All you have to do is:
Select the number, right click it then choose "send WhatsApp to this number", your browser will automatically direct you to https://web.whatsapp.com/, click continue and you will be ready to talk to the person you want to contact.
If the number is from your contry, simply select it (the code will be automatically added).
If it's outside your country, you will need to select the code along with the number.