I replied to a similar question, and I am re-pasting my answer here also.
Whatever value the metadata has for OBFILE will ultimately be the file name.
Example –
# The fileset driver’s configuration in NetConfig
# may be overridden by setting the equivelant key
# to the desired value in the messaga meta data
# field DRIVERCTL.
#
# Here the name of the outbound field is changed
# from what is configured in NetConfig to the
# value supplied by the user by setting file name to
# the value currently in the variable ‘file_name’.
set dvr_ctl_str “{FILESET {{OBFILE $file_name}}}”
msgmetaset $mh DRIVERCTL $dvr_ctl_str
