You will want it to be in a Canvas panel,
then the widget can get its slot, its slot is where all its position data goes for placing it within the Canvas. you do this in the event graph of the UMG widget.
once you get the slot, you need to cast it to the right type of slot, so if its in a canvus then cast it to a canvas planel slot,
after that you can drag pins away and set position, zorder, anchors etc 
Hope that helps.