How can I add a polygon and change the opacity?

You can import shape files in KML format or you can draw your polygons manually. For KML import check: KML files import. If you wish to draw a polygon on your dynamic map, you can access the 'Add features' section and go to polygons tab. Start shaping your polygon by clicking on the map.

Polygon Once you have selected the polygon area, you can further adjust the shape of your polygon, change the color and the opacity of your polygon. The option will appear when you click on 'done'.