Hi David,
Welcome to our support forums, and thanks for reaching out. Unfortunately this isn’t something that comes out-of-the-box with the Filter Bar plugin right now. We are aware this is something many users need, and it’s actually planned for upcoming releases.
That being said, it’s possible to get this working but will require some coding on your end. You can take a look at this sample, also explore this thread and this other thread. That should get you on the right track.
Hope you can get this customization up and running soon,
Best,
Nico
PS: You can up-vote and subscribe to this feature at our UserVoice page.