If no path is provided here Jenkins will use the latest installed QF-Test version.
In case you want to use a dedicated QF-Test version, please provide the respective path, e.g.
C:\Program Files (x86)\qfs\qftest\qftest-<VERSION>\
This version will be the default for all of your QF-Test jobs.
However there is an option to overwrite this version in the advanced job configuration.