If you want them to sleep on the roost, you probably have to put them on it.
But you can just let them sleep on the floor. If you provide a roost, they will start by playing on it in the daytime and continuing to sleep on the floor, but eventually they will start roosting at night all by themselves. Some batches of chicks start this sooner than others: I've seen some that were roosting at night by about a month old, and others that weren't roosting until they were almost old enough to lay eggs, and plenty in between. Those ages are what I've seen in chicks that were raised in a pen with no older chickens. Being in a pen with older chickens seems able to change the age up or down: they see the example of the older ones roosting so they may want to start at a younger age, but the older chickens may chase them off the roost which can keep them on the floor until they are older.
