How to add combobox selections to listbox

I need a page where i have a listbox and a combobox filled with choices.
There will be a link called “Add” that will add the selection in the
combobox to the listbox. Then when the page is submitted, the controller
collects the items in the listbox.

I have tried fiddling with collect_select and select, but did not get

Any help is appreciated!