Every user interface needs shortcut keys associated with it. To program these into your UMG interface, you can simply wire-up certain key combinations to an Action mapping. When the Action triggers, just invoke the same Blueprints function that the UI button itself triggers.
In Settings | Project Settings | Input, define a new Action mapping for your hot key event, for example,
HotKey_UIButton_Spell
.Wire up the event to your UI's function call either in Blueprints or in C++ code.