There are a few options.
With most dpak packages, you can simply give the slug (tab) the same number as the pin it is connected to. (This is how it is done in the official lib. TO-252-3_TabPin2 connects the tab to the middle pin which is what you want here.)
Or you can add a second pin to the symbol. (either visible and next to the other pin or invisible and on top of the other pin. Depends on your preference) There is however no fitting footprint for this in the official lib. (TO-252-4 should be called TO-252-5)