Shopify standalone app authentication

I am developing a standalone app using Shopify APIs and I’m quite stuck at the authentication step.

My app customer can currently authenticate with email/pwd and Google (social) but how do I allow them to authenticate with Shopify access token?

I’m using Auth0 a 3rd party library for authentication and there isn’t any integration between Shopify app and Auth0.

I want to ensure a merchant can signup automatically with Shopify…