Use only alpha value to control transparency of regular rectangular mask

[fmatulic]

I have an image as sole child of a regular rectangular mask and I just want to control the level of transparency of the whole image/mask. Is this possible with SoftMaskForUGUI? When I converted my Unity mask to Soft Mask my image turned all white and I wasn't able to get it to appear by fiddling with the options exposed in the inspector.