I don't think I understand this one.
"Why not just assign the btnloadclick-procedure as the event for the OnDblClickk-event in the events-list of the listbox? "
In Form-Designer design your Form (e.g. put a Button and a Listbox on it).
click on your Button for it to get the focus.
Go to the Properties-Panel, there on the Events-Tab, look for the OnClick-Event, mark it, then click on the Button with 3 dots "..." to create the default-Procedure for that event.
Write the code for the Button-Click-Event.
After that done, click on your Listbox, go to properties-panel, there on the Events-Tab, look for the OnDblClick-Event, but instead of clicking on the 3-dot-button you click on the dropdownlist there.
There now should be your Button-Click-Procedure to choose.
Choose it.
Done!