First, you need to create a free Userbase admin account. No credit card required.
Then, you need to include the Userbase SDK in your web app.
You can either include the SDK with a
Or, you can use the npm package:
npm install userbase-js
You can check the browser compatibility list here.
In your admin account, you will find a Trial app. Get its App ID, and initialize the Userbase SDK with it.