Accessing logged in user's username(credentials)

Hi, I need to add an sso_api to my current shopify store which will redirect user to an external site. In my understanding there is no way to call an api without creating an app and you then call the api through the app. The API i need to call requires the current logged in user’s email address in order to redirect it to an external website. Is there a way to get the current session’s username(email address) from shopify?