You have to pin a node as underlay to view it when you edit your mask. Right click the node you want to mask, and select "pin as underlay". The rest is same as before, click the button "edit mask" from the properties bar on the right. Once you draw a mask, just plug the output to the node you pinned earlier. You can unpin the node once you have finished editing your mask.
The input was just for visualization purpose, a remnant from older versions. Now all nodes use the pin feature for visualization, no need for a separate input.