Here an example of my base URL
<base href="http://subdomain.domain.com/folder/" />
I would like to know how to call that base URL for the HOME link when I try
<a href="/">Home</a>
It redirect to http://subdomain.domain.com/ and not in the /folder/
What you need is the virtual directory name of your application/website.
This is a server variable, and it is not possible to get it in HTML alone, you need some server side component, like PHP or ASP.NET.
If you are in the root directory, you can try a relative link.
So try using a . to indicate "from this folder"
<a href="./">Home</a>
Should your question actually concern the opposite direction, do this:
<a href="../">Home</a>
Or use an absolute link:
<a href="/folder">Home</a>
Or use a canonical link
<a href="http://subdomain.domain.com/folder">Home</a>