The Xiaomi Smart Doorbell 3S and the chime have been paired before leaving the factory, and the chime can be used directly after being powered on.
For your reference, you can pair the Xiaomi Smart Doorbell 3S with the indoor chime as follows:
Go to the [Mi Home/Xiaomi Home] APP >> select [Xiaomi Smart Doorbell 3S] >> tap the three-dot icon at the upper right corner >> [Doorbell settings] >> [Indoor receiver settings] >> [Connect to doorbell receiver] >> [Connect]. After the chime is powered on, press and hold the pairing button for 3 seconds and select it when you hear a beep sound. Then tap [Next] >> press the doorbell button >> [Send pairing code]. When the chime rings, the connection is successful.