Back button for social media authentication page?

I need to implement a back button for social media auth pages. For example - when I tap the FB or Google button I'm redirected to an external credentials and accept screen. If I want to go back to my app and I'm using android device - it's ok, because I can handle the hardware back button. However - when I'm using a iOs device - it's a bit different as I only have one button.

Any ideas how to solve this?

Frameworks in my app - angularjs and Ionic.