It's a little annoying for me to press F12 each time during PXE Boot as I do testing all the time.
Here are the steps to skip F12 :
On your WDS server(s) navigate to
Make a copy of your pxeboot.com (the version that prompts you to press F12)
Delete the original pxeboot.com
Make a copy of pxeboot.n12 (the version that bypasses the additional F12 prompt)
Rename this copy to pxeboot.com
Flag this file as READ-ONLY. If you do not do this the next reboot of your server will revert this file back.
Restart your WDS service and away you go.
Here is an additional article about the detail boot files: