The most prevalent messaging app whatsapp brought three ways to message a number(chat) on WhatsApp without adding them as a contact first.
The first one is WhatsApp’s own wa.me links; WhatsApp provides an official way to contact an unsaved number. You can start a chat with any number by going to the web address https://wa.me/phonenumber. Replace the last part with the full number, with the country and region code, but without any + or – or () or 00.
The second one is Use an app to speed things up; You simply enter the phone number, tap a button, and you’re taken to a newly-created chat in WhatsApp.
Among these apps, Click to chat requires no permissions to speak of. Direct Message for WhatsApp and Easy Message are a couple of other good alternatives without ads and with few permissions.
The third method may be the lesser known of the bunch. Whenever you have a phone number in any app, you can simply select the corresponding text, and check the suggested actions.
The main ones are always Call, Cut, Copy, Paste, but if you tap the overflow button (three dots), you may see more options including sending a WhatsApp message. Just select that and the chat will be started.
It’s not clear why the limitation has to exist inside WhatsApp or why you can’t simply type a number and message them like on any SMS app, but here we have at least three ways to bypass that restriction.