Use the Keyer Channel to extract a key from a red, green, or blue channel, or from a custom value.
To access the Keyer Channel menu, use:
Batch, then select a node from the Node bin.
Timeline, then use Batch FX (see Creating Batch FX).
The Modular Keyer, then select a node from the Node bin.
This node accepts a front clip, and outputs a result.