If you use a normal set up of leader -> swivel -> split ring -> lure, then just attach the solid ring from the assist to the split ring.
For some lures though, you will find this method of rigging works best with as little weight as possible pulling the nose down. In those cases, uni-knot the leader straight to the lure and loop the assist (minus the solid ring) through the towing point of the lure.
Which popper did you see without a belly swivel or wire hook point? It may be that this lure is specifically balanced to be used with a tail hook only.