First of all you should have your icons and splashscreen in place
(in the source link is explained really well how to do it)
Then set applicationID in package.json (usually domain in reverse order,
example: org.nativescript.yourexample)
You need an Apple Developer account because you need a production
certificate and a distribution provisioning profile
$ tns build ios --release --for-device
In Xcode select the project and in the Dropdown menu in the middle
select build for "Generic iOS device"
In the top menu under "Products" click archive
Click the blue button "Distribut app"
Go through the guided steps