If you convert a Windows Install ISO to a .imgPTN partition image file, you can automate the install by adding an 'answer file'.
If you wish to add an answer file, you must add it to the correct place inside the .imgPTN file.
If your E2B USB drive is a 'Removable' USB Flash drive or you have a Helper Flash drive, place your AutoUnattend.xml file in the root \ inside the .imgPTN image.
If your E2B USB drive is NOT of the 'Removable' type (not a 'Removable' USB Flash drive), you must use \Sources\AutoUnattend.xml and add it into the boot.wim file (see below) - OR - for more flexibility see below for details which allows you to easily change the answer file without needing to edit the boot.wim file every time - OR - also connect a small Removable-type USB Flash drive with your AutoUnattend.xml file in the root of the drive before you boot from the .imgPTN file.
The XML answer file should contain a valid WindowsPE section or else it will be ignored.
Instructions for adding an answer file to a Windows Vista/7/8/10 boot.wim file
Assuming you have made a .imgPTN file from a Vista/7/8 Install ISO and you want to add your own unattend.xml answer file...
1. Boot from your E2B USB drive (using a real system, RMPrepUSB QEMU or VirtualBox+DavidB's VMUB utility) and switch to the .imgPTN image
2. Unplug and reconnect the E2B USB drive - you should now see contents of the image
4. Copy the \sources\boot.wim from the E2B drive to your system hard drive (e.g. D:\boot.wim)
5. Create an empty folder called MOUNT on your systems hard drive (e.g. D:\Mount) - the folder must be empty.
6. Run GImageX as Admin and select the Mount tab. Set the Mount Point and Source, click Select... and select the Setup boot image from the boot.wim file (usually no. 2) - tick Read and Write and click on Mount. See 1 below...
7. Now use Explorer to add in MySetup.cmd and winpeshl.ini to the \Mount\Windows\System32 folder of the mounted image. If it already contains a winpeshl.ini file, then combine the two ini files into one so that mysetup.cmd is run from the [LaunchApps] section. The two new files are here.
8. Now save the changes by ticking the Commit Changes box, select the mounted image in the large Unmount window and then click Unmount. See 2 in the above screenshot
9. Finally delete the \Sources\boot.wim file from the E2B USB drive (hold down SHIFT and press DEL in Explorer so that it is not copied to the Recycle Bin) and then copy the new D:\boot.wim to the \Sources folder on the E2B USB drive.
10. Now you can add either a Unattend.xml or a AutoUnattend.xml file to the root of the E2B partition. e.g. if E: was the E2B partition, you should have these new files in the E2B image paritition...
It is now easy to change the answer file without needing to edit the boot.wim file.
See also here for a similar method that allows you to prompt the user to choose an XML file (and thus choose different Product Keys or configurations).