Hello,
Set up wind in the UE4 or Games versions of the SpeedTree Modeler, and then use export to game to make the .st file to import into UE4. The importer will set all the correct info into the materials for you.
Here are some relevant docs pages to cover each of those steps. Hope this helps
http://docs8.speedtree.com/modeler/doku.php?id=qsue4