In a development environment, (and some productions environments) you can simply use http://localhost, but if you are using any Authentication, you are going to need some sort of callback for tokens and secrets to be sent to.
And, if I may, if you’re just now starting to develop an app on iOS, I would probably not use Kraken endpoints. that entire side of the API has been deprecated and is scheduled for removal in December 2018. Check out the docs for Helix. It may seem a bit slim right now compared to Kraken, but they’re actively working on improving it.