How to train in Columbus > Find Texture?

Hi, I am trying to use Columbus' "Find Texture" module by training, but I must be missing something. I setup the building block, and then click on the "Train..." button , but nothing happens when I try and then click on the image. Is there some other trick besides left-click? This just works as a pixel classifier, right? I don't necessarily need to use upstream objects or measurements, do I?

Screenshot of workflow: https://cl.ly/3O1T170l0J1I

Thanks for any advice.

David