How to change the XFade on a transition node in script?

Question

I'm currently learning Godot and my textbook seems to be a bit out of date as it still uses the deprecated AnimationTreePlayer. So I've had to figure out AnimationTree on my own. The exercise for this lesson talks about changing the XFade for the transition node to a random value when the player presses the "up" key but I cannot for the life of me figure out how to do it. I see in the documentation that AnimationNodeTransition has a set_cross_fade_time(value) method but how do I get to it? How do I even access the tranisition node in the script? I've tried things like $AnimationTree/Transition and $AnimationTree["parameters/Transition"] but nothing I've tried works.

My script is currently on the root node and the node tree looks like this:

• Sprite [script is here]
  • AnimationPlayer
  • AnimationTree