Go to Pricing Plans and then to All Pricing Plans.

For each unit, you need to create a different pricing plan where you can assign it to the OTA or channel of your choice.
For example: If you have 3 units and each is listed on Airbnb, Booking, and Expedia, you must create a pricing plan for each channel, including direct bookings or your website.

Save the pricing plans and connect them in the channel manager to map the rate with the unit on the OTAs.