App doesn't redirect back into shopify /admin/ dashboard on install

[drbenway6667]

Hey bro,

any idea on how to get the app to redirect back into shopify e.g *.myshopify.com/admin/app-name/ on install? In it's current state it redirects to the site externally where the app is being hosted. e.g https://testserver.com/connect not back to the shopify admin dashboard inside the iframe as you would expect.
When I try and redirect it back to the shopify admin dashboard /admin/app-name/ via html, http redirect & shopify bridge js something breaks and I get the following message:

Oauth error invalid_request: The authorization code was not found or was already used

any pointers would be most appreciated

Cheers,
henry
str8up.media