Didn't work for me, either. Nor for one of the commenters. I can see a use in a mobile browser, but URLMPro handles this on the Desktop without even having a browser open! Of course, there are several ways to "share" a site/url/etc. on most mobile devices.
I only tried dropping a url into a DropBox tab, in Safari. It might work with another window open.
All it did in the tab-drag attempt was to change the DropBox tab to another tad of the site I was dragging. At least it also allowed the back button to permit getting back to the DropBox page... oh well...