WhatMe is an add-free app that allows you to start WhatsApp & Message chats without saving the contact you need to message to.
With WhatMe you can select your country of preference in order to automatically add the country code to the phone number you want to message.
WhatMe keeps a history of the phone numbers you have wrote to in case you need them in the future. You can also add records on your history to your contacts if you want.