Here the simpeline

REALMNT="/mnt/media_rw/extSdCard"
mount -t ext4 -o rw,noatime /dev/block/mmcblk1p1 "$REALMNT"
sdcard "$REALMNT" /storage/extSdCard 1023 1023 &

You can change "extSdCard" like "sdcard1" where your external sdcard mounted

Iam used stockrom with Kingroot, but I try Supersu its trouble with mounting filesystem