Is there a particular reason we cant bind this? I would like to set it to custom % based on game events.
Hey @Ninja4848! Welcome to the forums!
So you CAN do this- the thing is, that is covered by the “Set Opacity” node in the event graph, so it actually functions more like a non-UMG element!
You just plug in the element you want to set the opacity for, and it does the rest.
Try this thread here for more ideas!
Thats close but not exactly what i was hoping for. That suggests using ‘bind on hover’. i was hoping for a bind on variable/Function - like all the other elements have. i do have a work around, but its much more annoying then regular binds.
Yes, that thread was just for more ideas- not to copy them. You can switch out that event for anything… say, a dispatcher from wherever you’re trying to do this. 
Or a Blueprint Interface!
The thing to take away from the link is all the code after the first node- the “Set Opacity” node is, effectively, a binding.
I believe it is not able to be set as a binding due to the native code.