For Xiaomi it’s a bit different indeed, as the App page already mentions:
Xiaomi does not officially support controlling devices from outside the Mi Home app. Additional steps are required for pairing these devices with Homey that require some technical knowledge. It also means not all available devices are supported. Also be aware that Xiaomi can make changes to it’s eco system at any time resulting in Homey not being able to control these devices anymore.
Please visit the support topic for supported devices, installation instructions and further details.
In the history of the Community Forum topic, i’ve found references to these pages for installation instructions:
Pairing miIO devices
For Homey to be able to communicate with devices over the miIO protocol a unique device token needs to be obtained. In some cases technical knowledge is needed for retrieving these tokens. See the instructions here on retrieving device tokens or here - YouTube and here for which driver to pair your device with.
Edit: Welcome to Homey by the way 
