Hi Lish - select your rectangle, and then run the Construct / Fillet command.

Some points will appear at each corner, and the prompt in the upper-right corner of the window will say "Select corners, or push done to do all".

At this stage, you can either push Done (or right-click in a viewport) to apply the fillet to all corners of the curve, or if you only want to do just one corner, select just that one and push done.

Next you pick a radius just like you did for rounding the box - either click 2 points and the radius will be the distance between those points, or you can enter an exact one if you want.

Hope this helps, let me know if you need more info!

- Michael