How to allow an external application to connect to any number of Shopify stores

[djachim]

We have an external application that will need to listen for transactions from any number of stores owned by our clients. The requested solution is to allow a client to log into the external application and log into their Shopify store so that the store returns credentials to the external application so that it can retrieve the data collected by the store.

Is there a generic url that my external application can redirect to (to allow a client to log into their Shopify account and choose the Store) to return my external application so it call APIs to retrieve data from the Store via a permanent token or credentials?