From Xojo Documentation
Rectangle is used to display rectangles and rounded rectangles on the layout. The user does not typically interact with this control.
BorderColor - The color to draw the border.
BorderThickness - The width (or thickness) of the border.
CornerSize - The corner height for drawing rounded rectangles.
FillColor - The color for the fill area of the rectangle.
This code in the Opening event handler updates the rectangle to have thick blue borders, rounded corners and a red fill: