Button to add URL parameter chosen by dropdown menu

I have a dropdown menu (named mydropdown) that lists a series of parameters (value1, value2, value3), then I have a button that should open a url to which the parameters chosen in the dropdown must be added (https://myurl.com?param=value1).
I don't understand how to set "Custom URL parameters" in the button settings, I tried {data:mydropdown} and various combinations but always without success, what should I do?
Thanks in advance.

The same for me. It seems that doesn't work.

same here..