Apple tree shoots
If the tree is a cultivated variety, the fruitful top has probably been grafted to a root stock. If the shoot is growing below the graft it is part of the root stock and propagating it will produce a tree with the root stock's characteristics, not those of the top.
Successful grafting requires skill and sometimes luck. The following bulletin describes the process.
If you decide to proceed and do the job yourself, we recommend taking a class to learn the basics, or asking a professional to help. Someone at a local orchard may have the skills needed to propagate the tree.