In the iDEAL TransactionResponse, an IssuerAuthenticationURL is provided to you to which you should redirect the user is to be redirected. This URL will lead the user to the banking pages (browser or mobile app) in which the user can authenticate the iDEAL payment.
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It is important that the provided URL is used for the redirect without adjusting any of the content! This includes any encoding of the URL before using it for the redirect, as this may break the URL (parameters).