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How to retrieve parameters from return_url?

I am trying to implement “Paypal Onboard for Sellers”.
User clicks “Connect Paypal”, then he is directed to “Log In with Paypal”, and finally he comes back to my return_url.

The problem is that at the end of the day, I don’t understand how to retrieve authCode and sharedId from this process.
Paypal literally says

When your seller completes the sign-up flow, PayPal returns an authCode and sharedId to your seller’s browser.
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What is “returns to browser” ? Nobody knows…

here is the Paypal instructions Build onboarding into your software