You can use the commandline parameter: -deskmode
E.g. make a copy of your existing desktop shortcut to BS.Player.
In "properties" of that copy add at the end of its "target":
[space]-deskmode.
Now start BS.Player using that copy shortcut.
Start movie.
(Of course BS.Player's rendering mode must be suitable for desktop-mode)
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Read possibilities in the file "cmdline.txt" in the folder "doc" inside BS.Player's installation folder for further options.
E.g. to automatically start with a specific movie use as commandline:
"[MyDrive]:\[MyPath2Player]\bsplayer.exe" "[MyDrive]:\[MyPath2File]\MyMovieFile.avi" -deskmode