How to send message to injected script?

Question

JavaScript

I am making for myself an extension for auto-completing fields on the site. The logic is as follows: when the page is loaded, a script is embedded that will change the fields, an account is selected in the popup extension, after which the popup should send data to the embedded script. Actually the question is: how to send and how to listen to messages between the extension and the embedded script.

Answer 1

postMessage. How do you read docs?