You set up your recording rule's "Apply these tags (optional)" like this:
Field: Filename (path)
Value: C:\TV Recordings\Flash\Season [Season]\
Is "Flash" the name of a TV series? If so, you can even use this (so it will work for any TV series):
C:\TV Recordings\[Series]\Season [Season]\
However, since it is specified in a recording rule, which most likely is set up for a particular series, it should be fine just use the hard-coded "Flash".