Cannot run a Task Sequence as the Local Administrator

I would like to Run a Task Sequence Run Command as the Local Administrator, as opposed to running it in the Local System Context.  Is it necessary to specify details of the local Administrator account in the "Run this Step as the Following Account" field?  If so, I notice that it needs to be in the format of Domain\User or %Variable%.  What should the %Variable% value be?

Any ideas?