How to sent (fetch) data from js to node.js

I want to make a register function

I try to do like this:

function finish() {

    if (valid) {
        const postData = {
          email: email.value,
          username: username.value,
          password: password.value,
          password2: password2.value
        };
      
        fetch('/register', {
            method: 'POST',
            headers: {
                "Content-type": "aplication/json"
            },
            body:JSON.stringify(postData)
          });
    }
}

and in node js i have this:

app.post('/register', async (req, res) => {
  try {
    const email = req.body.email;
    const newuser = req.body.username;
    const password = req.body.password;
    const repetpasword = req.body.password2;
    console.log(newuser);
    if (checkvalidinput(email, newuser, password, repetpasword)) {
      res.render('startpage')
    } else {
      const message = 'sumphing is wrong'
      res.render('errormessage' , {message: message});
    }

  } catch (error) {
    console.log(error);
  }

});

when i try to display in console the variable “newuser” is dispaly “undefined”, how can i solve that?
Any other idea, other metod forsent data from js to node.js?