Attaching guide wire to guide wire for patio string lights
Visit your local hardware store and find whatever loops or rings they have that are cheap and easy to work with--ideally something that can be installed on the cable after it is installed. Even those cheap keychain carabiners would do well, at least until the hinge pins rust away.
The rings will self-center to some degree, which is what you want. You may have to secure the ends to the posts somehow to keep them from sliding. Small eye hooks might work for that.
More specific answers are probably not helpful since we don't know what your store will have.