React - Handling forms and submitting POST data to API -- @see: https://reactjs.org/docs/forms.html

api-form-submit.js

class NameForm extends React.Component {
  constructor(props) {
    super(props);
    this.state = { name: '' };
  }

  handleChange = (event) => {
    this.setState({[event.target.name]: event.target.value});
  }

  handleSubmit = (event) => {
    alert('A form was submitted: ' + this.state);

    fetch('https://your-node-server-here.com/api/endpoint', {
        method: 'POST',
        // We convert the React state to JSON and send it as the POST body
        body: JSON.stringify(this.state)
      }).then(function(response) {
        console.log(response)
        return response.json();
      });

    event.preventDefault();
}

  render() {
    return (
      <form onSubmit={this.handleSubmit}>
        <label>
          Name:
          <input type="text" value={this.state.value} name="name" onChange={this.handleChange} />
        </label>
        <input type="submit" value="Submit" />
      </form>
    );
  }
}