Any hybrid identity journey starts with verifying your DNS domain name in Azure AD. This recipe explains how to do this.
You'll need an Azure AD tenant for this recipe. If your organization doesn't have a tenant, sign up for one using https://signup.microsoft.com/Signup?OfferId=B07A1127-DE83-4a6d-9F85-2C104BDAE8B4.
The form provides a way to name the Azure AD tenant. This is a plus, because many other signup methods base the name of the tenant on the email address of the person signing up. The preceding link creates a non-expiring tenant with a 90-day Microsoft Office 365 E3 trial license. You specify an account with the Global administrator role during signup.