Start Action

This allows you to open a default file every time you start debugging.
Save the workbook to a local folder
Manifest Project
Add > Existing Item
Select the file
This will add the item to the Start Action drop-down.
Choose this file in the drop-down

Custom Start Document

Right click on the manifest project
Choose Add Existing Item
Browse to your start file
Change the "start document" to refer to this file
When you launch this file from inside Visual Studio, your add-in will not be automatically loaded.
Insert > Add-ins > My Add-ins
Select your add-in
Save the file and close it and stop debugging
This will save the add-in lanch details in your workbook
Next time you launch from Visual Studio your add-in will be loaded.

Start Document

Renamed from Start Document in Visual Studio 2017
Allows you to specify a test document that will be loaded every time you start debugging
Add the test file to the top project