How to use Super-L instead of Alt in XFCE for resizing/moving windows

In XFCE, when holding down Alt and then clicking anywhere in a window with the left button will move that window (as if by dragging the title bar), or resize the windows when clicking and dragging with the right button.

I like this feature very much, and I'm used to it, since forever. But I've always had it bound to Super-L(left "Windows"-Key).

How do I change the behavior of Alt for Super-L? I was looking at XFCE Settings Manager but I couldn't find it anywhere.