Until Etsy approve our app we need to obtain user account details from Etsy stores that the app will publish products to.  The screen shot below shows how to see the ID number of your Etsy user account.

Browse to the Etsy Shop Manager.  Press F12 in the browser to show the Tools window and then select the Elements tab from the top rail.  The ID number is displayed in the left-hand pane, highlighted by the red arrow.