How to create an INSTANCE_SWAP prop on a component?

I figured it out. The INSTANCE_SWAP property required the id of a ComponentNode or a ComponentSetNode of the instance you are attaching it to:

const newComponent = figma.createComponent();
const instance = componentSetNode.defaultVariant.createInstance();

newComponent.appendChild(instance);
instance.componentPropertyReferences = {
  mainComponent: newComponent.addComponentProperty(
    "PropertyName",
    "INSTANCE_SWAP",
    componentSetNode.defaultVariant.id
  ),
};

// if you start from an instance and do not have access to its original ComponentNode:
newComponent.appendChild(instance);
instance.componentPropertyReferences = {
  mainComponent: newComponent.addComponentProperty(
    "PropertyName",
    "INSTANCE_SWAP",
    instance.mainComponent.id
  ),
};