I had a similar problem as described in this thread while logging WSJTX QSOs through JTAlert. After following the directions in this thread, I was finally able to get the 'address' field in the QSL box of DXKeeper's 'Log QSOs' page to fill automatically. The key direction, found in "step 2.e in the JTAlert section" of the link Dave provided above, was in part iii, "select the Logging section in the left-side panel and consider each of the options that govern XML lookups". Checking the "Log Address returned from an XML or previous QSO lookup" box did the trick.
I hope this helps others who might be in the same boat. And thanks to Dave and the others above who helped me to get this working as I wanted.