How to crop a rectangle with rounded corners.

[PixLab]

One of the way I do it is with the Rectangle Select tool   , then in the option tool I tick the "Rounded corners" a slider appears below, play with it
then menu > Select / Invert > hit that delete key or add a mask (I prefer the mask)
   

Also you can do Ctrl+A, then Ctrl+C then go in the menu Select / Rounded rectangle... a window opens input your percentage, then like above > Select/Invert > Mask or delete