Login and register for specific multisite

Hi guys,
I’m having a little problem that I cant seem to find a solution to.
I have a multi-site, and I want my users to be able to sign in to their site, from a global homesite.

What I want is a extra login field (can be a drop-down but I prefer that they just type the name of their multisite) on my WP login site.

I have created a image to help explain what I’m after.

https://www.dropbox.com/s/grjp5oqlpfh585a/Login.png

The best thing is if someone knows about a plugin that has this feature, I have looked very hard but cant find it.

Anyone who can assist me on this matter?

Many thanks in advanced!!