Fetch: Can you pass parameters to the server

I'm using a basic fetch to obtain data from an express server that queries the data from a database. So I want to do a login system, i want to send the user/password from the input fields with the fetch request. so i can perform the logical check to see if password matches the data in the server. and respond with the result. Is it possible to do a fetch that can pass parameters?

Updated Code Below:

let fetchData = {
  method: "post",
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json'
  },

  body: JSON.stringify({
    username: this.state.user,
    password: this.state.password
  })
}

var fromServer = fetch('http://localhost:3000/', fetchData)
.then(function(response){
  if( !response.ok){
    throw Error (response.statusText);
  }
  return response.json();
})
.then(function(response) {
  console.log(response);
})
.catch(error => console.log("there was an error --> " + error));

Express function

app.post('/', function(req, res){
  var uname = req.body.username;
  var pw = req.body.password;
  console.log(uname);
  console.log(pw);
});