How does one create a certificate request for a managed service account in Windows?

I have a managed service account which needs a certificate in its personal store for decryption. I tried opening the Certificates snap-in and pointing to the service, but when I right-click on the "Personal" store the Request New Certificate option is not available. I only have Import... and Advanced Operations > Create Custom Request.

I tried creating a custom request in a number of different ways, but the certificate comes back with my user DN listed in the subject field, and the private key ends up in my current user's Certificate Enrollment Requests store.

I'd rather not abandon using a managed service account, but at this point it appears that I might have to. Any insights would be greatly appreciated.