This documentation shows you the menu, and what you'll want to do is click 'color to alpha' where you can enter the color code (get it with the eyedrop tool) that you want to send to alpha (transparent)
This documentation shows you the menu, and what you'll want to do is click 'color to alpha' where you can enter the color code (get it with the eyedrop tool) that you want to send to alpha (transparent)