Even if you could append a parameter of your liking and choice, to the oAuth flow, you’d perpetuate a bad architecture all because it was easy to do? Yay for progress!
I am not sure you can do this. First off, Shopify oAuth flow is out of your control. When you hit the Shopify oAuth, go right ahead. Attach any params you want!! See what you get back. Answer your own question! It takes seconds, since you already have that flow under your control in your App. Do a quick debug.