React JS - How to authenticate credentials via a fetch statement

My goal is to create a React JS login page that runs off a json Rest service. In Postman, when I enter the URL for the service, set it to run as POST and enter the following JSON into the body: {username: "myUserName", password: "myPassword"} ...a token is returned. So in my fetch clause, I'm using JSON.stringify to pass the username and password to the server.

I'm new to using Fetch with react, So my question is, how do I get started in authenticating various users, just using react JS with fetch only? I assume, I'm to write my logic within the second then of my Fetch clause?

Currently, my page accepts any credentials and routes the user to a landing page upon clicking the submit button. I have a function containing fetch and now calling the fetch function once the onSubmit button is clicked, which now grabs the token.

This is my code:

import React, { Component } from 'react';
import ReactDOM from 'react-dom';
import './Login.css';
import { withRouter } from 'react-router-dom';

class Login extends Component {

    constructor() {
        super();
        this.state = {
            data: [],
            username: "",
            password: "",
            token: "",
        };
    } //end constructor

    componentWillMount() {
    }

    componentDidMount() {
        this.fetchData();
    }

    fetchData() {
        fetch('http://theapi/api/auth', {
            method: 'POST',
            headers: {
                'Content-type': 'application/json',
            },
             body: JSON.stringify({
                username: 'myUserName',
                password: 'myPassword',
                Authorization: 'TheReturnedToken',
            })
        }) /*end fetch */
        .then(results => results.json())
        .then(data => this.setState({ data: data })

        )
    }

    //request the token
      requestAccessToken(data) {
        const loginInfo = '${data}&grant_type=password';
        return fetch('${API_URL}Token', {
          method: 'POST',
          headers: new Headers({
            'Content-Type': 'application/json',
          }),
          body: loginInfo,
        })
          .then((response) => response.json());
      }

      //authenticate request
      requestUserInfo(token) {
        return fetch('${API_URL}api/participant/userinfo', {
          method: 'GET',
          headers: new Headers({
            Authorization: 'Bearer ${token}',
          }),
        })
          .then((response) => response.json());
      }

    change = (e) => {
        this.setState({
            [e.target.name]: e.target.value
        });
    }; //end change

    onSubmit = (e) =>{
        this.fetchData();
        e.preventDefault();
        //console.log(this.state);
        this.setState({
             username: "",
             password: "",
            });

        this.props.history.push('/landing');
        };

    render() {
    console.log(this.state.data);
        return (
           <div>
                <div className="loginContainer">
                <h2>Member Login</h2>
                    <form>
                            <input
                            id="username"
                            name="username"
                            placeholder="User Name"
                            value={this.state.username}
                            onChange={e => this.change(e) }
                            className="form-control"
                            />  <br />

                            <input
                            id="password"
                            name="password"
                            type="password"
                            placeholder="Password"
                            value={this.state.password}
                            onChange={e => this.change(e) }
                            className="form-control"
                            />  <br />

                        <button onClick={e => this.onSubmit(e)} className="btn btn-primary">Submit</button>
                        </form>
                    </div>
            </div>
        );
      }
}

export default withRouter(Login);

How do I get started in getting my form to authenticate various users? Basically, I'm attempting to have my page to accept a username and password and if the two match, and then route the user to a landing page.