The Briqpay JS SDK is used to communicate with the iframe, listen to events and validate changes. It is also what automatically adjust the iframe's height to fit on your page, along with more necessary communication.
The SDK has to be loaded for all Briqpay iframes and is automaticly included in the session html snippet as a script tag. If you use client-side rendering you might need to load the script in your document head instead. See Loading the client for more information.