This is the example implementing custom headers for WebView requests.
(I used alexziskind1/nativescript-oauth as reference).
├── dist // built files
├── index.d.ts // type definition
├── index.ios.ts
├── index.android.js
└── sample // sample application
cd sample
tns install
tns platform add [ios | android]
tns run [ios | android]
Now, you can see your customized headers using Safari on iOS, Chrome on Android.