So, let's start with downloading Inventor HSM Express (sorry Autodesk...you may get hit with a bunch of form submissions). (and if you need it here is a link to the download http://cam.autodesk.com/download/inventor-hsm-express/)
Once downloaded install the software and then we can get down to business with setting up the external NC Editor. Side note: if you have HSMWorks then the same concept works; you will just need to find the HSMWorks folder with the editor.
Inside of Fusion; hit the pulldown under your name and click on preferences.
Now, on the CAM selection the very top option is for an External Editor, pick on the button to the right of the text box as seen in image.
this opens a dialog box that allows you to path to an executable file (in our case the HSM Editor that was installed with the express software). Here is the path that will take you to the executable: C:\Program Files\Autodesk\Inventor HSM 2016\Editor
The one thing that I am not sure of is the "Inventor HSM 2016" file folder...it might have express on it, but not particularly sure on that one.
Anyways, once you find that Editor folder pick on the InventorHSMEdit.exe and pick the Open button.
Hit OK on the Preferences dialog box and post away; now when you post you will get the NC Editor rather than the Brackets program.