Using a 3rd-Party Certificate with WAC

I would like to use a 3rd-party certificate with WAC and select it during the installation.  We can request one from our cert provider DigiCert, but I don't know how to create the cert request for WAC.  Since WAC isn't installed yet, I can't create a request FROM WAC.  How would I create a cert request for DigiCert on behalf of WAC?