Add UnattendJoin component to your Specialize pass, like so :
<component name="Microsoft-Windows-UnattendedJoin" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<Identification>
<Credentials>
<Domain>domain.com</Domain>
<Password>yourpassword</Password>
<Username>yourusername</Username>
</Credentials>
<JoinDomain>doma