How to execute Javascript on someone else's site?

Question

JavaScript

Good day dear comrades.
There is an online store, when you click on the button to add any product to the cart in the browser inspector, I monitor the execution of the add.js javascript, after which the product is added to the cart, I would like to know:
Can I send such a js request to the site myself from a self-written application so that it would work as well?
If so, please help with knowledge, an example, advice or a link to materials.
Heavenly thanks in advance!

Answer 1

You write userscript. You install a browser plugin that runs userscript. You start. This is how I caught the appearance of the product and added it to the cart. Since a lot of people hunted for the goods. Then an API appeared on that site. (but it doesn't matter to me anymore)

Answer 2

It is not very clear what you want to use the technology Web service
is now the most popular RESTful API