When you invoke the Hook window, it needs to identify the current document, resource, or context. If Hook cannot identify the current context there are three likely explanations.
A new unsaved document
It is impossible to link to (or from) a document until it is saved. Some applications (e.g. Notes.app, Bear, nvALT) save new documents as soon as they’re created, but many applications (e.g. TextEdit, BBEdit, Photoshop) let you open a new window and start writing or modifying new documents which don’t exist in the file system and cannot be hooked until they are saved.
Save new documents before trying to hook them.
Document is not selected or open in the frontmost window
The context is determined by the contents of the frontmost window. If another window is in front, Hook won’t identify the correct context.
Ensure that the correct window is frontmost, and that the desired document is open or selected.
Application is unsupported
Most macOS applications work with Hook but some require special support for their integration.
Learn more about integrating Hook with unsupported applications.