It's actually more important, in terms of the domain continuing to work properly, that your clients and member servers use Domain Controllers for their time source than it is that your clients have the correct time. I know this sounds odd, but it's true. Clients on a domain need their clocks to be similar within a certain degree of accuracy in order for clients and servers to trust one another for authentication, etc.
Therefore, my suggestion is that you leave the clients and member servers syncing time with the domain, and configure the DC holding the PDC emulator role to be the one that uses the external time source.
There's a possibility here to save the licence you're using for the dedicated ntp server and have the PDC emulator sync directly to time.windows.com itself, by the way. The "local dedicated ntp server" scenario is more common when your local ntp server is a stratum 1 time server connecting directly to a reference clock - for example I use two of these.
Anyway, to configure any system to use an external time source, try something like the w32tm.exe command:
w32tm.exe /config /manualpeerlist:”your.ntp.server.fqdn” /syncfromflags:manual /reliable:YES /update
To force a manual update from your newly configured time source, to check it's working and make the initial changeover:
w32tm.exe /config /update
Restart the time service for the change to take effect:
From the command line / batch script:
net stop w32time
net start w32time
Or if you've been using the powershell environment: