When using fabric softener on your laundry you should run the laundry detergent first and let the laundry get washed and then during the rinse cycle you add the fabric softener.
You're not supposed to add fabric softener with the laundry detergent because it may not wash out as good.
Fabric softener is only meant to soften clothes and laundry and doesn't wash the laundry.
So if your washing machine has a fabric softener dispenser put the fabric softener in the dispenser and the laundry detergent in the detergent dispenser or into the drum.
Just make sure the fabric softener is in the fabric softener dispenser and it will be released into the washing machine during the rinse cycle.
If you don't have a fabric softener dispenser you add the fabric softener during the rinse cycle.