How to send a WhatsApp message to a number that is not in your contacts
It is annoying to have to add a number to your contacts if you only want to send a one-off message or to contact that person temporarily. A quick workaround is enter the phone number in a message to yourself. WhatsApp will create that number as a link so then you can just click on it and WhatsApp will give you the option to chat, call or add to contacts.