This works for me. You need to make sure that if you're pointing to a dashboard that you also explicitly specify the sheet on the dashboard that you want to edit.
My example url for a dashboard looks like this:
Let me know if this is the issue you were running into.
(What is the error/behavior that you're getting?)
Yes, I think my initial issue was that I was not passing a specific sheet name.
Then I ran into a 404 page not found. But the issue for that is for some reasons, the "Edit" was not enabled on the workbook. Now it works, thanks!