Making Custom variables available in Config form

I've got a site set up which uses the domain access module to manage 5 different sites. Long story short, I have a taxonomy structure which spans all the sites, but have created a custom module and variable where I can set a parent term ID for reference by certain blocks on the sites.

Got my module up and running, variables are being set just fine, however it's only globally at the moment, and I want it to be configured per domain - I've got domain variable on the site, but I can't configure my custom variable to be available to the domain realm, because it's not listed on admin/config/system/variable/realm/domain/configure...

So basically the question is, how do I make my variable show up here, so that I can configure it per domain? I've presumably have not used the right hook to set my variable, but I can't find a whole lot of documentation around about how to use Variable Realms, so thought it would be quicker just to ask for support.

Thanks