Open file in existing emacs frame (Windows)

I accidentally figured this out while trying to fix synctex with SumatraPDF. It would appear that in addition to the ALTERNATE_EDITOR environment variable pointing to runemacs.exe, you must also create an EMACS_SERVER_FILE environment variable that points to the server file (mine was stored in the .emacs.d\server directory). Once I did that, files that I tell to Open with Emacs opened in the existing frame rather than creating their own.