Microsoft recommends having an on-prem Exchange Server if you want to manage your users through your on-prem Active Directory. Because of this and also because they know many organizations are still using quite old versions of Exchange that do not work with Office 365, they offer a free license for Exchange 2013. Here is the catch: no mailbox capability for the on-prem server, only ease of management tasks. But I’ll take it! Download the appropriate (when this article was written) Exchange 2013 version here.
To access your license key, login to your Office 365 tenant as a global administrator. Go to this address using the same browser to reach the Exchange Hybrid Product Key Distribution page to verify eligibility and get your key.
If the above link does not work, select Setup on the admin page of the 365 portal.
Under Extend Your Setup, select Advanced Setup and follow the questionnaire for hybrid deployments. They system will generate you a key eventually.
Even easier, you can contact support through your portal and they will provide you with the key assigned to your organization. The serial can be used for multiple installs of Exchange 2013! Hope this helps…