Pannier racks usually have a loop or ring of some kind near the bottom of the main leg. Some have 2 legs which are welded together then separate to make a V-notch for these tension systems.
Here's a link to a
bunch of images showing the diversity of these arrangements. However some racks, especially ones not made specifically for panniers may not have anything. In that case, you can buy or improvise small
eye hooks that you'll attach under the head of the mounting bolts.