How to use Global OAuth clients? Especialy How to capture subdomain of Zedesk account?

Hello,

I have an application that uses Zendesk API and I had it set up Global OAuth clients to get access to other Zendesk accounts to let my application access their APIs. 

As of now, I did set up Global OAuth Clients and as per the instructions from the email I received it says, I have to capture the subdomain and do some validation before I redirect to a Zendesk account to complete the OAuth process. This is part I did not understand and there is no documentation or guide on How to use the Global OAuth clients after setting it up. Such as this one Link .

If there is any set of instructions/documentation on how to capture subdomain during global OAuth clients please let me know, That would be great. 
Thank you!