Create a url that automatically adds blog_path

Anyone know a way to create a url link that automatically puts in the multisite blog_path into the url?
I have an affiliate link that uses the blog_path of each blog as the reference name. Like this:
http://mysite.com?ref=blog_path

But I haven't figured out how to automatically add the blog_path the the end of the url. And have it stay so I can put it into my Simple Ads.

Use: I want to have each blog to have a image banner that will be linked to the url by adding the blog_path to the end which is the same as their affiliate reference.

Anyone know how to do this?

Kindly,

Paul