PublicKeyCredential.rawId

rawId = publicKeyCredential.rawId

var options = {
  challenge: new Uint8Array(26) /* from the server */,
  rp: {
    name: "Example CORP",
    id  : "login.example.com"
  },
  user: {
    id: new Uint8Array(26), /* To be changed for each user */
    name: "[email protected]",
    displayName: "John Doe",
  },
  pubKeyCredParams: [
    {
      type: "public-key",
      alg: -7
    }
  ]
};

navigator.credentials.create({  publicKey: options })
  .then(function (pubKeyCredential) {
    var rawId = pubKeyCredential.rawId;
    // Do something with rawId
}).catch(function (err) {
  // Deal with any error
});