Generally, if we want to send a WhatsApp message to any number registered on WhatsApp we have to start a chat from the contacts page. This requires us to have that particular contact already saved in our phone. Unlike the traditional SMS service, using the WhatsApp app, we cannot just enter a number and start a chat with an unknown/unsaved contact, while the reverse works, i.e. we can receive chats from an unknown number.
However, there is a trick using which this can be made possible.
Enter this URL in a browser, replacing the CC with the country dialing code without any symbol (91 for India) and the XXXXXXXXXX with the 10-digit phone number you wish to start a conversation with.
wa.me/CCXXXXXXXXXX
for eg:
wa.me/919876543210
This URL then expands to :
https://api.whatsapp.com/send/?phone=919876543210
This is basically a WhatsApp API, so this method is completely safe and not a 3rd party plugin.
If this URL is sent as a WhatsApp message, then simply clicking on it will start a chat with the desired number.
If doing this from a PC
- Make sure you are logged on to web.whatsapp.com or the WhatsApp desktop app.
- Then click on the Continue to Chat (if using WhatsApp web) or Open WhatsApp Desktop (if logged on the Desktop App) button and you are good to go.

Whatsapp Send Message to Unknown Number
Using this you can send a message to yourselves too for making a grocery list or creating such draft messages.