It's such an obvious action that there has to be a message which handles it. I want to be able to click on a number in the tallyroll. If we do, it must be transported to the entryfield. In doing so, it'll also delete the current entry. Because this action has a destructive effect on the already-entered number, we must take the approach that this can't be done accidentally, so a single click on a number won't do.
First thing is to find a way to detect a double click on the listbox.
A double click is a typical event happening on a control, in this case the listbox. Inside this we'll have to handle the click. For obtaining this index there is a convenient macro available, the WinQueryLboxSelectedItem macro. After that we can query the contents. Now that we've got the contents of a line, we need to think up a way to process them.