We have explained some of these tricks.
Open any web browser on your phone. Next type the link "http://wa.me/91xxxxxxxxxx" and press enter.
After typing the number, press enter to open the link. You will be redirected to WhatsApp screen. Click on the green button which says "Continue Chat". The WhatsApp chat window of the entered mobile number will open. Now you send them the message.
Open the Apple Shortcuts app on your iPhone. Tap on the "Add shortcut" button.
Now install the WhatsApp to Non-Contact shortcut. Once the shortcut is installed, tap on it to run it. A pop up saying "Choose recipient" will appear.
Type the number with the country code (+91- for Indian number) in the "Choose recipient". The WhatsApp chat thread of the particular number will open and you will be able to message the person.