Mesh Reference to Widget ?

How can I reference a Mesh to a widget Blueprint ? It´s easy with a Level Blueprint where I just need to drag and drop an Mesh and get my instance. But I can´t seem to get it done in a widget. I allready tried to create a Static Mesh Reference where I selecetd the desired Mesh. But it´s somewhat different from dragging a Mesh to the Level Blueprint. Basically I want to change a Material by pressing a button. Following this tutorial it´s easy to do by pressing a key. Unreal 4 Changing Materials in Blueprints - YouTube I want to achive this by pressing a button