Post-setup commands are added in the setupcomplete.cmd file, not unattended.
You can invoke that file with Unattended, if OEM keys are used (toolbar option), but that's a separate topic.
Is the image isolated, outside of the ISO structure, like a single WIM file, or you are loading the entire ISO content?
To get the creation of .\autounattend.xml, make sure to use the full ISO content, and apply changes.
If it still doesn't work, please attach the preset and tell me what to look for.
Thanks.