Multiselect checkboxes example

I’m referring to your previous topic: https://www.ads-software.com/support/topic/multiselect-checkboxes/.

I’m trying to add up the values of options in a checkbox group:

[checkbox numbers “2” “3” “9” “15”]
[calculation totalnumbers “numbers * 1” ]

But the result remains zero, even if I just select one option.

How to do this, and how to trigger the calculation automatically for any tick or untick, without have to use a calculate button?