Need some help with drag and drop operations

I am currently creating a tower defense game. I have a grid set up that will be where my weapons get placed, as well as a umg widget that has images of all the weapons available to place. What I need is for the player to be able to click on one of the weapons in the umg, and have the ability to drag said weapon into the grid where it would be placed to auto-fire at enemies once the enemies step into the weapons direct path. Any ideas how to do this? I tried doing a drag and drop operation, but that didn’t work.