Hello wbielski

It seems that Dell have changed their own mount and you can only use the AW3423DW with the MSA20 if both the screen and monitor arm use the respective VESA adapters that are included.
So it should work, but it adds a few cm to the depth and doesn't look quite as elegant as without.

I would probably buy the arm anyway, as I have one and can tell you that it is really good ;-)

Best regards
Andrew