Removing "press any key" prompts for GPT/UEFI Windows install automation
Have you ever tried automating a Windows install using GUID Partition Table (GPT) partitioning, only to discover that you always get a "Press any key to boot" prompt that you can't disable?

You're not missing something. It turns out that Windows install images behave differently when you are using UEFI/GPT vs. legacy BIOS on boot[1]. By default UEFI/GPT displays a prompt, legacy BIOS boots directly to Windows install. The behaviour is not consistent. Apparently this was a default that was carried over from Intel Itanium (IA-64) architecture before the engineers fully understood how UEFI was going to be used in the real world.
Workaround
So how do you change this behaviour? Well, you need to make a new boot disk. All Windows install media comes with a second "no prompt" bootloader, it's just not enabled by default:

What you'll need
To create a new boot ISO, you'll need the following:
- Windows machine with the Windows Assessment and Deployment Kit (Windows ADK) installed
- Enough free disk space to hold two copies of a Windows boot ISO
Steps
1) Mount the Windows install ISO
2) Copy the Windows install ISO to a local disk so you can change it:
3) Modify the local copy of the Windows install ISO in c:\iso
to use cdboot_noprompt.efi
and efisys_noprompt.bin
bootloaders:
4) Start an elevated deployment and imaging tools environment (created when you installed the Windows ADK).
Choose Run As Administrator
via Deployment and Imaging Tools Environment
under Windows Kits
in the Start menu to launch the imaging tools environment:

5) Use the oscdimg
tool to create a new boot image. Be patient - it will take several minutes:
6) Use your _noprompt.iso
with impunity - look ma, no prompt!
References
[1] https://blogs.technet.microsoft.com/jhoward/2013/11/11/hyper-v-generation-2-virtual-machines-part-9/