Redirect to Firebase Hosting custom domain

You cannot turn off the subdomain. Your app will always be available on https://myapp.firebaseapp.com and whatever custom domain you've set up.

To redirect people, you can add a canonical link to your HTML:

<link rel="canonical" href="http://myapp.domain.com/" />

Read more about that in Specify your canonical on the Google Webmaster Central Blog.

You could use Firebase Functions.

Free for 125K invocations/month - https://firebase.google.com/pricing

An example using Express middleware:

// functions/index.js

const functions = require('firebase-functions');
const express = require('express');
const url = require('url');

const app = express();

// Allowed domains
let domains = ['localhost:5000', 'example.com'];
// Base URL to redirect
let baseurl = 'https://example.com/';

// Redirect middleware
app.use((req, res, next) => {
    if (!domains.includes(req.headers['x-forwarded-host'])) {
        return res.status(301).redirect(url.resolve(baseurl, req.path.replace(/^\/+/, "")));
    }
    return next();
});

// Dynamically route static html files
app.get('/', (req, res) => {
    return res.sendFile('index.html', { root: './html' });
});

// ...

// 404 middleware
app.use((req, res) => {
    return res.status(404).sendFile('404.html', { root: './html' });
});

// Export redirect function
exports.redirectFunc = functions.https.onRequest(app);

The exported function name must be added to rewrites in firebase.json e.g.:

{
    "hosting": {
        "public": "public",
        "rewrites": [
          {
            "source": "**",
            "function": "redirectFunc"
          }
        ]
    }
}

In addition to specifying canonical link as mentioned in Frank van Puffelen's answer. We can also add front end JavaScript code to do the actual redirect like this without disclosing default url.

if (location.hostname.indexOf('custom.url') === -1) {
    location.replace("https://custom.url");
}