I would make the MID in the Construction Script the way Enos shows above, and you can right click on the Return Value, and Promote it to a Variable. Then, you can just change this Variable in the Event Graph with a Set Vector Parameter. You can see lots of examples of this in the Tappy Chicken example game.
Also, in your material you can just run the red directly into the Emissive channel only, if your material type is Unlit. Also, you don’t need to multiply it by the Mask. That will just darken it around the edges.