How Do I Append a Character to the End of a Photo Name via Batch?

Question

NOTE: I tried to put in several tags like PowerRename, PowerToys, Batch, Process, Rename...nothing comes up. So I put something random in here to get it going - Teams. Any help on this would be appreciated, too.

This is a pretty simple question, and I feel like I'm just on the verge of finding it - yet I can't.

I have a group of photos that I want to append an "a" to them by renaming. I've used PowerRename, and it's fantastic, but I'm not quite versed on the syntax to make this happen.

For example, I want to take Park 001.jpg and rename it Park 001a.jpg. All of the photos are already correct on their numbers, so I want to keep those intact (e.g., Park 009.jpg, Park 017.jpg, and Park 121.jgp to Park 009a.jpg, Park 017a.jpg, and Park 121a.jpg).

I've started with .{8}(.) and realized that the (.) isn't needed. I've selected "Use Regular Expressions" and tried a combination of ${} and $1 with no luck. I'm close...just no cigar.