This extension requires you to have https://play.google.com/apps/com.codedamn.whatsappAutoMessages installed on your phone. If you don't know what Scheduler For WhatsApp is on Android, you probably shouldn't install this extension.
APP LINK: https://play.google.com/store/apps/details?id=com.codedamn.whatsappAutoMessages
SCHEDULER FOR WHATSAPP
The above android app allows you to schedule messages on whatsapp. Anytime, anywhere. Works even in lock screen. However, you need to install this extension to start the app (just first time).
HOW TO USE
Please see the video on left for clear explanation.