When using the Parameter Expression option, it's unclear what the expression needs to resolve to. After some guessing, I figured out it likes credentials IDs. First, the fact that I had to figure this out by trial and error is annoying (it's undocumented AFAICT).
Second, it would be nice if you could search by name/description. Otherwise, for this parameter to be used, I have to maintain a table of name to ID mappings somewhere in the job's parameters, and that's fragile and redundant.