Passing data from a React Native App to iOS and Android

I quite finish to build my app and I'm using an external solution to get push notifications and analytics (in the native code).

I have to send them the email of the user when logged to "allow" notifications. I tried it in a static way, and it works like a charm.

Android exemple :

HashMap contactData = new HashMap();contactData.put("f_EMail", "email@gmail.com");

But now, I need to find a way to send the email stored in React Native App to iOS Xcode and Android Studio. In a more dynamic way

I read the documentation about the bridge between react native and native but I didn't understand everything. I read some topics here but no one seems to have found the solution.

Did someone can help me ?

Thank you