Problems copying masks between layers

I'm fairly new to Photoshop, but have been making great progress. I am stumped at the moment as to how to copy a mask to another layer particularly when that layer may be an Adjustment Layer. The steps I follow are:

1. On one layer, create a selection and output that selection to a layer mask of the layer from which I select. (Or a similar approach)
2. Add a Curves Adjustment layer for which I want to apply only to the same selection as the selection in Step 1.
3. When I do CMD + click on the mask and then drag to the Curves Adjustment layer, I get a prompt, 'Do you want to replace the existing mask' or a message to that effect. I say yes and the mask gets moved rather than copied as I understand the behavior should be.

I am running PS 2019 on Mac Mojave OS.

I'm sure there is some little nuance that I'm not doing correctly, but I can't for the life of me figure it out.

Note that I experience the same behavior if I try to Cmd + Click drag to a regular layer.

David