Sending JavaScript commands to an external site

I am looking to make a website that would allow an user to either buy or sell steam items. For this I need to be able to send steam trade offers and manage item inventories. I have figured out how to complete trade offers using the chrome console sending JavaScript commands, but I need to know how to do this on my website instead securely.

Is there any way to send JavaScript commands to an external site behind the scenes? If so, would I be able to do it on a basic web hosting service's server?