Check out the “Drag & Drop” section. There is a tutorial that explains how to move a health bar on the screen with the mouse, that should give you some idea about coordinate translations with regard to viewport. it is quite complex.
The other thing you may want to look into is raycast in order to hit an object with the mouse. I wager that might contain some Blueprint nodes you can use to determine the 3D world position from the mouse position.
Once you have a 3D position that is virtually behind your button, you should be able to start some particles from there.
I don’t think that you can start particles “in the 2D realm of UMG”. They are a 3D element and are part of the 3D world. You will run into problems if the camera moves while particles are flying.
