Even though WhatsApp has existed for over ten years now, the messaging app still only lets us chat with our contacts. If you want to reach out to a new person, you need to first save them to your contact list on your phone, wait for WhatsApp to sync so that new name shows up inside the app, then start a chat with them. This usually takes several minutes and could often go on for hours if the contact doesn't sync quickly.
If you want to bypass this silly restriction and immediately chat with a new contact, there are several tricks for that. One of them is provided by WhatsApp, but it's not accessible inside the app for some reason. If you're not aware of these, I'll explain three of the easiest methods below.
WhatsApp's own wa.me links
WhatsApp provides an official way to contact an unsaved number, just not within the app. As explained in this FAQ, you can start a chat with any number by opening Chrome or Firefox or the Samsung browser, and 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.
So for example, if you want to chat with the number +1-234-567-8900 in the US, you would go to the URL: https://wa.me/12345678900. Type it in the browser and you'll get the web page below with a big green Message icon. Tapping on it opens a conversation with that phone number in WhatsApp (if the number is registered on the service, obviously).
Use an app to speed things up
If the above method seems a little confusing or slow for you, you can use an app to speed things up. Several Android apps use these wa.me links but in a non-user-facing way — i.e. you don't have to type a URL or pass by a web browser first. You simply enter the phone number, tap a button, and you're taken to a newly-created chat in WhatsApp.
Text selection and App Actions
The third method may be the lesser known of the bunch but it's the easiest, fastest, and best integrated. 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.
To try it, you can easily type +phonenumber (including the + and country code at the beginning) in the Google search widget on your homescreen, then tap to select it, and tap the three-dot button. An option to send a WhatsApp message should pop up if you have a compatible device.
In our testing, this works on both Android 9 Pie and 10, but only on stock devices (Pixels and Android One). We saw the other actions on Huawei, Samsung, and OnePlus phones, but not the WhatsApp one. Hopefully, the integration will make its way to non-stock Android devices later.
These workarounds should come in handy if you frequently have short-term chats with strangers. For example, I sometimes have to talk to people to arrange selling or buying some second-hand items, and I don't really need to add all these temporary acquaintances to my contacts. I'm sure you can think of, uhm, other ways this would be useful.
It's not clear why the limitation has to exist inside WhatsApp in the first place, or why you can't simply type a number and message them like on any SMS app, but here we are. Thankfully, we all now have at least three ways to bypass that restriction.