You will have to check pretty quickly after the action runs otherwise there is a chance the agent will cleanup the file.
You can look in the agent log to see that it’s writing the file out, on the delete __createfile line you should see this in the log:
Command succeeded delete No 'C:\Program Files (x86)\BigFix Enterprise\BES Client\__BESData\a site of yours\__createfile'
and the wifi.xml should be in that directory