Firebase: push notification by API

i am trying to call the firebase cloud messages API like this:

    fetch('https://fcm.googleapis.com/fcm/send',
    {
      method: 'POST',
      headers: {
      //  'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
        'Content-Type': 'application/json',
        'Authorization': 'key=AA##A8RX######Kt-SrCn##eXu'
      },
      body: {
        "notification":{
            "title":"note-Title",
            "body":"note-Body"
            },
        "to":"??????"
        }

      
    })

Console output:

POST https://fcm.googleapis.com/fcm/send 400 ()

honestly i have no idea what i have to insert after “to”.
I want to send the push notification to all users.

Please help.