How do I make a new benefit?

I was poking around in my rewards tiers and I have several different tiers levels. I have benefits for some. I must have forgotten how to make a benefit because I have made them in the past but I don’t know how now. In my tiers I can see an “add benefit” button. I can add a mug which I made a long time ago. I need to make a new benefit but I don’t know how to do this. Can anyone tell me how to make a new benefit?