Is the only way to add a quicklaunch in LXQt by mouse?

I try to add a quicklaunch item to the panel in Lubuntu 18.10. This seems only to work with drag and drop from the start menu and not via a menu for the panel setting.

That's correct; as of LXQt 0.13 there is no way to modify this within the LXQt Panel settings; it's mouse only.

Sorry, it should read like this:

[quicklaunch]

alignment=Left

apps\1\desktop=/usr/share/applications/pcmanfm-qt.desktop

apps\2\desktop=/usr/share/applications/firefox.desktop

apps\3\desktop=/usr/share/applications/qterminal.desktop 

apps\size=3 

type=quicklaunch

This is how I did it in Lubuntu 19.04 to get the qterminal into the panel by changing the quicklaunch-section in the file "~/.config/lxqt/panel.conf":

[quicklaunch] alignment=Left
apps\1\desktop=/usr/share/applications/pcmanfm-qt.desktop
apps\2\desktop=/usr/share/applications/firefox.desktop
apps\3\desktop=/usr/share/applications/qterminal.desktop 
apps\size=3
> type=quicklaunch