I want to get to the FiddlerScript editor without using the mouse but I don't see a shortcut for this in https://docs.telerik.com/fiddler/KnowledgeBase/Keyboard.
From Fiddler Reissue and Edit and Reissue from composer:
It's ctrl+R.
However, this opens the FiddlerScript editor as a separate window, which isn't exactly the intent of the question, but I find it acceptable for my purposes.