The third parameter to the function ("toggle") is a boolean; make it "true" if you want to add;
SearchColumn(colNo, searchString, toggle)
This seems odd to me, I'm not clear on what 'toggle' means:
The documentation says "Whether to toggle selections."
I thought it means if "TX" is selected, and you SearchColumn(0, "TX", true) again, the selection will turn off
I guess it (also?) means that selections are cumulative; i.e. like holding Ctrl.
I used this property of the click event to as the toggle parameter: