Automating Website Sign-In With HTTP Requests

Disclaimer: I am new to web programming and server requests.

I am creating a Chrome extension to automate sign-in to a site. The website requires a username and email, and my script so far has been loading the page, filling in the values with Javascript, and then clicking the "Login" button.

Is there a way to do this with HTTP requests? Is there a way I can send the browser the relevant information without loading the whole page, and if so, how do I figure out where and which information to send?

(I realize this is very insecure, forget that for now.)