The description of Open with WhatsApp - WhatsLookup
WhatsLookup app uses whatsapp's offical public api to open a chat with any number without saving the contact on your phone's address book.
How does this app helps? You received a call or message or notification from an unknown number and you wanted to know if they have whatsapp to decide yourself, all you have to insert the country prefix and phone number in the application and click for lookup. App will open the unknown number in WhatsApp - This works without saving the contact on your phone's address book and you can begin chatting with unknown number and keep track of recent history.