If I understand you correctly, you don't want the subs to be on the black bar below the movie, but you want them to be shown on the movie-surface itself.
If so, please use auto-position and try the following settings:
(rightclick >) Options > Preferences > Subtitles [Subtitle properties]
Deselect/uncheck "Enable drawing subtitle under image with EVR renderer"
and select/check: "Draw subtitles to Overlay surface"
(I suppose "Use new subtitle parser/renderer" is selected?)
Another option/possibility to get the result you want is also available there by changing the value of "Subtitle margins bottom (%)". Maybe experiment a bit with that setting.
P.S.:
Note this is also depending on what video rendering you use under
(rightclick >) Options > Preferences > Video > [Video rendering].
So, if the above isn't working please provide further info on i.a. rendering mode and OS.