hello
i made a tutorial about referencing widgets and tiring to put it in the wiki when am done
check it out it may help you
http://q8debug.com/create-umg-using-blueprint-and-c/
i would recommend for your your situation (as i understand you have a button on your widget is done in the editor and you want it to do some functionality on event click)
make a function in C++ and make it BlueprintCallable
then set up the on-click event in editor for your button then just connect it to the C++ function this is the way i do it.
make visuals and events in UMG and functions in C++.