Mathcarver,
Rather than try to tie in a loop on poppers you’ve already completed, there is a simple ‘fix’, if you will, that will keep the knot on the hook bend from slipping down and off of your barbless hooks.
Just wrap a snug ‘bump’ of tying thread on the bend of the hook. Add a drop of head cement, let it cure, and you have a ‘stopper’ that will prevent the dropper knot from sliding off on you.
Works fine, and is quick and esy to do.
Good Luck!
Buddy