Header keys become lowercase when interacting with API - React native

When interacting with API, the custom Header key always become lowercase. I'm use Fetch, Axios, XMLHttpRequest and Frisbee (javascript network library) but the key always lowercase

My snippet code like this (with fetch() method), The key I'm push is: 'Token-Api' but the server receive: 'token-api', so it's show the error 401. It's work with Postman:

 const request = 'https://abcxyz';
 fetch(request, {
   method: 'GET',
   headers: {
     'Content-Type': 'application/json',
     'Token-Api':'...abcxyz....'
   }
 }).then((response) => {
            log(abcxyz)
        })...

What do I need to do?