Hi Nancy According to the website of the manufacturer of Rootone (a commonly used rooting hormone), their product is only for ornamentals. If you are using another product, check the label or call the manufacturer for more information.
Luckily tomaotes root pretty well on their own. Take the sucker and put it in a pot of moistened potting soil. Keep the soil moist and the pot out of direct sun and in a few days the sucker should be rooted.