The body parameters are not working in fetch post react native - api

Question

I have a post request from react native app to chragebee for creating new customer, in postman it works fine, and in the app it works fine also, but the problem is in passing the body parameters from the app to chargebee, so i can not see for example the body parameters in the response body return from chargebee like the email, firstname, lastname , .... after creating user from the app, whereas in postman i can see them.
The postman post request
The response in postman
The response in postman if i remove the body parameters
The function responsible for signup process in react native :
export const ChargebeeCreateUser = (first_name,last_name,email) => {
const bodyParams = {
first_name: first_name,
last_name:last_name,
email:email,
};
return fetch(`https://editpress-test.chargebee.com/api/v2/customers`, {
method: 'POST',
headers: new Headers({
'Authorization': 'Basic ' + encode('site_api_key'),
'Content-Type': 'application/json'
}),
body: JSON.stringify(bodyParams)
})
.then((response) => response.json())
.then(responseJson => {
return responseJson
})
}
Calling the function:
register = async () => {
const { given_name,family_name,email, password } = this.state
let ChargebeeCreateUserResponse = await
ChargebeeCreateUser(given_name,family_name,email);
console.log(ChargebeeCreateUserResponse);
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The result of console.log(ChargebeeCreateUserResponse):