Transform transform { get; }

Implementing this interface in a UnityEngine.MonoBehaviour will automatically inherit this property so you do not need to do anything else:

public class MyComponent : MonoBehaviour, IAnimancerRoot
{
}

But if you want to have your script point to a different object as the root, you can explicitly implement this property:

public class MyComponent : MonoBehaviour, IAnimancerRoot
{
    Transform IAnimancerRoot.transform => ???;
}