Augmented Reality in TV Production with Blackmagic DeckLink Duo 2 - I´m searching for the option to get the "Key" out of my Unreal Engine 4.27. I saw the "Fill" , but I miss the "Key".

Development Cinematics & Media Virtual Production
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I configured my setup with a Trackmen Tracking System, in a classic TV-studio. There I want to place some AR-objects. For this I need a Fill&Key-signal. In the UE I Configured everything like it is shown here: Blackmagic Video I/O Quick Start | Unreal Engine Documentation
Now I am searching for the channel, output etc. for the “Key2-part”. Can anyone help me with this?