From Xojo Documentation
Draws a rounded rectangle filled with the DrawingColor.
x and y are the coordinates of the top-left corner. width and height specify the size of the round rectangle. ArcWidth controls the shape of the corners of both the horizontal and vertical axes. It is the distance (in pixels) from the corner at which the arc begins. Setting ArcWidth to zero results in a rectangle with sharp corners (a normal rectangle).
This example draws two red rounded squares. The left square sets the arcs to 10 and the right one to 25.
g.FillRoundRectangle(10, 10, 100, 100, 10)
g.FillRoundRectangle(130, 10, 100, 100, 25)