the screen is not auto generated, but part of the platform actually (probably this one: file-edit.xml). Generally you can extend screens when you have a subclass of FileDescriptor e.g.: https://doc.cuba-platform.com/manual-6.5/screen_extension.html, but i’m not really sure, that this is the right thing to do. Can you elaborate a little on what you want to achieve?
Instead of using the default edit action, you could also do and do whatever you want in
myMethod - this will probably lead to a better position, depending on what you want to achieve.