How to Create a Whatsapp Web Chat Extension

I want to create an extension for Whatsapp web. The extension reads the message and then sends it to the JS script does some operation and returns the output. A very common example is the Grammarly extension for chrome which reads the message and corrects your grammar in the sentence, I want to do exactly the same thing but my script won't be correcting grammar, it will remove some part of the text message as soon as the user clicks on the button (just like Grammarly). Can you suggest me, how to read the messages from the WhatsApp web and perform the action. Currently, I am building this extension for me and my friend as it is required for a project.