This works OK.
NLRECS tells it to stop after 1 record
You have to start REXX and use the FLD function to act like a SUBSTR
The SET PAD=ON makes it pass the line with spaces instead of low-values.
//FILEMGR EXEC PGM=FMNMAIN
//SYSPRINT DD SYSOUT=*
//FMNTSPRT DD SYSOUT=*
//SYSTERM DD SYSOUT=*
//DDIN DD *
LINE1
LINE2
LINE3
//DDOUT DD SYSOUT=*
//SYSIN DD *
$$FILEM SET PAD=ON
$$FILEM DSC NLRECS=1,PROC=*
OUTREC = FLD(1,4)