Extended Shopify Flow App

We need to create a shopify app that extends the shopify flow basically from our current flow after it adds a customer tag we want it to check if the customer has an account on our International website. (Also Shopify) If they already have an account = Do Nothing If they do not have an account = Create Account and apply the same customer tags Comment from Shopify Support 'Unfortunately as you may of suspected there is no native functionality within flow to achieve what you are looking to do here. Basically you need a action to call an external shopify site to check against the unique customer identifier (email) and action something should there be or not. To do this, a private app will need to be created that will sit in the middle of your store and external stores'