The WINE_SIMULATE_WRITECOPY=1 %command% is the Steam launch option you set, with %command% meaning roughly "what Steam would do without any launch options set".

The whole process was a bit finicky and I did it a few month ago, but from what I remember it went something like this:

• Download battle.net installer
• Add it as non-Steam game to run it
• Locate the newly created prefix in Steam directory
• Add the Battle.net.exe in it as a non-Steam game, then remove the installer (not the other way around or the prefix will be deleted)