@ECHO off
setlocal enabledelayedexpansion

SET "_stri=abcdefghıj"
SET "_strf=c"
SET "_s=0"
CALL :instr %_stri%, %_strf%, %_s%
echo Found at %_result%. position >> fun.txt
goto :eof

:instr
SETLOCAL
SET "_strif=%1"
SET "_strff=%2"
SET "_sf=%3"
SET "_cf="
SET "_gf="
SET "_yf="
FOR /L %%a IN (%_sf%,1,10) DO (
    SET "_cf=%%a"
    SET "_gf=!_strif!:~!_cf!,1
    IF "!_gf!"=="!_strff!" GOTO ENDF
)
:ENDF
ENDLOCAL & SET _result=!_cf!