How to use Rectangle as an opacity mask

Question

I'm trying to make a Layer Mask for black area in my image,I have an image like this:

I draw an rectangle on it:

how can i make a mask that show following image(White area 100% transparent and Black area with mask color)

Answer 1

1. In any image editor select all white color and remove it.
2. Save image in PNG format.
3. Use it in XAML like an opacity mask.