Hello Rainer,
normally you don’t need a launch.json. Depending on what settings you need, look settings for the extension in VSCode (Menu File/Preferences/Settings), then select Workspace and on the left side you see the extensions, search for “Robot Code”. Every setting that starts with “Robot:” are settings that corresponds to the “robot” command line parameters. Here you have to set the right things.
If your settings are correct, than you can simply run/debug your tests directly from the green arrow before a test in the editor or from the test explorer view.
Right click on the green arrow, gives you a menu where you can select “Debug” instead of “Run”.
Daniel