Just give them the opportunity. Mine forage for a lot of their food as well as eat the chicken feed I offer them. I have a large garden so they get excess and stuff that we don't eat. I toss fruit in their run in season. I don't try to micromanage every bite they eat because I can't since they forage. I let them handle that. They are healthy, active, lay a lot of eggs, produce meat, and hatch and raise chicks. I let them be chickens.
Sometimes when I toss things from the garden or fruit trees into the run they eat it like it's ice cream and cake. Really go after it. Sometimes they ignore certain things. One group may really eat broccoli or chard leaves but not eat cabbage leaves. Or they may prefer cabbage. Each group is different but usually if I leave it out for them they will eventually try it and eat it.
A story. One time I was canning corn and gathered a yogurt container full of corn ear worms while shucking and silking it. I dumped that container of corn ear worms on a bare spot in the vicinity of a bunch of 10-week-old chicks. Step by step, inch by inch, they carefully approached that pile. A worm wiggled! Run away! Run away! But they did not run far. Very soon, step by step, inch by inch, they again started stalking that pile. A worm wiggled! Run away! Run away! This repeated two or three more times before a very brave cockerel grabbed a worm. That pile was eaten within less than a minute. All it took was that one brave cockerel to grab one.
So just make it available. One day it will start to be eaten.