A free app for Android, by Loapps.
Open Zap is a useful tool to open a WhatsApp chat without saving the number in your address book. It's very easy to use and allows you to start a conversation with a single click.
It works as follows:
Once you have downloaded it, you need to open WhatsApp on your device and then click on the icon on the top-right side of the screen. You will be asked to log in with your phone number.
Now you can use Open Zap to start a conversation without saving the number in your address book. Simply enter the number, choose the chat, click "Start conversation" and you will automatically be redirected to the WhatsApp chat.
