It shouldn't be long before they discover the perch and use it to launch from in their daily antics.
However, close to the time you plan on moving them into their permanent coop, begin putting them on the perch at night after it begins to get dark. Around four weeks of age, they should be sleeping in the dark since they no longer need a heat lamp.
As it gets too dark for them to see, you can put them on the perch to sleep, and they won't jump off. A larger diameter perch will be more comfortable by then. I use a two x two stick for a perch in the brooder, and in the coop, my girls have a four inch diameter tree limb to perch on.
Teaching them to roost on a perch at night before they are moved into the coop will avoid the problem of them getting addicted to sleeping in a pile on the floor, and by then the amount of poop they produce overnight can be prodigious! You don't want them sleeping on a pile of poop!
But you are far out ahead providing them with any kind of perch! Most people neglect this feature when putting together their brooders.
