What I have seen others do is embed the script in the actual autounattend XML file iteslf and you would have to look at their github page to reverse engineer it for yourself.

This is cschneegans' xml generator

I think there is also a way to edit the ISO image itself and have an $OEM folder or something like that, but it's been too long since I looked at it so you would have to research that.

A good tool I used for editing the ISO image is called "AnyBurn"

Stand by for the admin that tells me how my answer does not solve your problem
in 3, 2, . . .