-Create a polygon cube, then add a Bevel to it and adjust the settings to your liking.
or
-Create a polygon cube. Increase its length, width, and height segments to whatever is required. Try maybe 10 to start. Press '3' on the keyboard. Alternatively, instead of increasing the segments in the cube, you can add your own edgeloops only in the critical areas with the "Insert Edge Loop" tool.
And of course - if this was 3ds Max, then just press the "Chamfer Box" button. 🙂