May we use Multipass to login *existing customers* on a Shopify store in a 3rd party web app?
e.g.
I signup on shop.com. Then I go to app.com and I provide the same email & password used on shop.com
How can app.com say if these credentials are valid, and get the Shopify customer id?
(app.com can already create customers with passwords, but don't know how to signin existing customers)
Thanks