lately CHRIS O’BRIEN write a great blog post about how to use your Azure Active Directory instance behind your office 365 tenant and he mentioned that the only work around is to subscribe and from his blog:
Using the Azure instance behind your Office 365 tenant
“Sign up for Windows Azure”
Yes, it’s counter-intuitive. As I mentioned earlier, the Azure instance already exists so why do we need to “sign-up”? Well, you just do – for now at least. I did hear a Microsoft speaker at SPC mention this experience as a side-note, and that they will probably improve things soon. The main thing to understand is that you have to supply payment details before this Azure instance can be accessed – because then it becomes a true pay-as-you-go instance.
but I discovered a workaround for this and its very easy.
2- Then here is the workaround now you need to choose use existing account
3- you will logout then log in with your Office 365 account then your AD will be added to be managed by this Azure instance
I hope that will save you the time you will need to sign up for Azure just to manage your Office 365 AD