Click on the left key and specify the Opacity as 10. Click on the right key and specify Opacity as 0. The result is shown in the lowest window. Click OK and return to the document. Click within the selection to create the initial fill.
Check results and make any required changes.
Delete or undo the previous fill before filling again. Create a new raster layer. Using the rounded rectangle selection, select an area for the highlight. Feather the section by 2 or three to blur the shape a little. Fill with white.