Answer
Fresh pre-killed prey is useful for very small slings that cannot safely tackle a live feeder. Many slings will scavenge once the food is left near the retreat.
What to use
Use a freshly killed small cricket, roach nymph, mealworm section or other established feeder. The portion should be small enough to remove easily.
How to offer it
Place the food close to the sling's retreat in the evening and check later. The sling does not need to attack immediately for the food to be useful.
Remove leftovers
Take out uneaten pieces before they dry, mould or attract mites. Tiny containers foul much faster than adult enclosures.
Move to live prey
As the sling grows and can subdue appropriately sized live prey confidently, live feeding becomes easier to monitor.