Hi @sanjin.o,
Thank you for your response.
After collaborating with other Engineers, I confirmed that it is not possible to obtain the query parameters with the Pre-User Registration Action. Our Engineers also informed me that this could present the risk of the end-user manipulating the URL.
If applicable for a workaround, you could try using an invitation flow by sending an email inviting the user to register. The invitation will allow the user to set their password and gain access to your application. A user invitation is essentially a change password link repurposed as an invitation. You can learn more about this in our Send Email Invitations for Application Signup documentation.
Please let me know if this works for you. Otherwise, could you please provide more clarity on your intended flow on allowing/denying users to sign up?
Thank you.