To achieve this you can either set up multiple rates with different minimum stays as explained here:
https://wiki.beds24.com/index.php?title=Minimum_Stay or use the "per night discount" in the "Discounts" tab of the rate. Please use the context help (click on the ?Icon)
This general help which opens when you click on HELP in the top right menu of the RATES page also has link to these functions.
Please note that discounts which are set in the "Discounts" tab of the rates can not be set to channels.