Authorization to Sign Form 5500 On Client's Behalf

If a client wanted to give our firm the authority to sign the Form 5500 on their behalf (I guess signing it under my ID), what do they need to sign to authorize that?  Would our firm just be listed as the Plan Administrator on the Form 5500-SF?

We've always had the client sign themselves, but for some reason one client doesn't want to do it and would rather have us sign.

Please see this post for more information --