How to add custom columns in SpiraTeam Test Sets?

This drop-down list allows you to change the fields that are displayed in the test set list as columns for the current project. To show a column that is not already displayed, simply select that column from the list of "Show…" column names and to hide an existing column, simply select that column from the list of "Hide…" column names. This is stored on a per-project basis, so you can have different display settings for each project that you are a member of. The fields can be any of the built-in fields or any of the custom properties set up by the project owner."

How do I add my own column to the Show/Hide columns list? Looking in Custom List and Custom Properties, but cannot find out where to add my own column.