The way a custom URL works is by replacing long link address bar strings by a short version that is customised to a name chosen by the person of the page owner. Another example is the +TOYOTA where this short custom url +toyota is being used for particular solutions in mind.
Nevertheless, currently Google has stated that only a small selection of verified profiles and pages have this facility at present. So, following this announcement we can only deduce this new tool will be available to all, within a few weeks. Feel Free to add your comments to this post, thank you.