Alt
+Shift
+DownKey
to box-select beginning of each string followed by Ctrl
+V
pastes only on the first line.
Expected behavior: paste my string on all selected lines.
Using Notepad++ v5.9
1) Mark
2) Push Alt+c
to open Column editor
3) Enter text there
As commented by @chapo it now works fine with Alt-Shift-Down
and then pasting.