Quixel Bridge for Unreal Engine 5 works with your Epic Games account and currently supports the Unreal Unlimited plan.
To log in, click on the icon in the top right corner of Quixel Bridge and select Sign In from the menu. You can log in using your existing Epic Games account, or sign up for a new account. To get free, unlimited access to the Megascans library, make sure you’re on the Unreal Unlimited plan.
You can find out more about the Unreal Unlimited plan here.