I know this post is kind of old, but there is an option available that doesn't necessarily require an AD account with password. Both realm and adcli commands support the --one-time-password argument, however this requires that the computer objects are pre-created in Active Directory and that you know the Machine Account Password of the computer object.
You can use the Powershell cmdlet Reset-ComputerMachinePassword and/or possibly the netdom reset or netdom resetpwd commands on Windows to manually set the Machine Account password to a well-known string. You can also use the Active Directory Users and Computers GUI to perform a Reset Account on the computer object in Active Directory - which changes the Machine Account password to match the computer object name (e.g. if the computer object is WORKSTATION01 this changes the Machine Account Password to the string "WORKSTATION01").
In such a case, you can then join Linux machines to the domain using the hostname (or whatever you set using Reset-ComputerMachinePassword) as the one-time password, without needing credentials to an actual domain user/service account.
realm join <mydomain>.example.com --one-time-password `hostname -s`