Where to add code that must go under the body tag?
 

I've recently had my forum approved for BuySellAds and I want to add their, code immediately underneath the body tag on my 3.8.4 forum. From the BuySellAds site;

Quote:
Main snippet that goes right after your opening tag.....

You only need to install the ... snippet ... ONCE for the entire website. DO NOT install this multiple times on your site.

What template do I need to edit, to add the code snippet immediately under the body tag?

Thanks!