IIRC, the cfg input files were still binary files back then, so if tab isn't working, you either have to delete the default cfg file, or test every button until you find it (and hope it wasn't remapped to "none").
Otherwise, it was basically the same as now:
Press the "Config Menu" (aka "UI Configure") button, whose default is the 'tab' key
which is (and was) usually shorted to "press the tab key".
