Transparent PNG Export

Im sure there must be something im missing here with the PNG export settings. I like the way the new export window appears but I cant figure out how to export from Draw to a PNG with a transparent matte of a color picked color.

How I did it in X4 was to chose to export to PNG, and when the window appeared to select a color for the transparent area, a color picker was used. Whatever I do with X5, i cant do it :/

Ive checked out the help files but I still cant figure it out... but im sure its something im not doing correctly.