Marco, there is no possibility to add the the list of user's forum topics neither using the Custom HTML, nor by creating a custom widget, because the user's forum topics list was not developed in the forum plugin. We have only the latest forum topics list which is placed on the Index page. To create a widget, you should first develop the user's forum topics list, then create component that will contain the list of topics and only after that you can create widget which will show a user's forum topics list.
I suggest that you download our new 'Plugin Skeleton' plugin to see how to use this or that function. Here is URL to the Store item:
http://www.oxwall.org/store/item/695