@bradsexton81 can you comment?
My suspicion is the need to do that in Custom configuration has probably been overcome by the options in the Profile area of the Image Library, where we specify the Deployment Final Action:
When the OS is installed, after Windows Setup completes there is an automated logon using the built-in Administrator account, which resumes the final steps (RunOnce) from MDT. This is the phase where the BigFix Client is installed, and there’s also a “Recover from Domain” step that runs in case the domain failed to join during Windows Setup…I’m not sure whether this issue still exists, but it was once a problem for me on Static-addressed machines, that during the image deployment, Windows Setup tried to join the domain before setting a static IP address, so my machines would always fail the “during-setup” domain join and would recover at this point.
Once this last step of MDT is complete, we can either reboot, log off, or leave the Administrator session open. Reboot is the default, and is definitely needed if the Domain Join just completed as part of “Recover from Domain”
For @zsmith1101 - My OSD lab is offline at the moment but I should be able to test this scenario next week. Just because several programming languages use the backslash as an escape character, can I ask whether you tried to specify your domain join credential as “DOMAIN\username” with two backslashes? That’s one of the test cases I’d try.