How do I stop losing a fragment of this url when redirected?

How do I stop losing a fragment of this url when redirected? References I\'m trying to parse callback url, and matching set of params out of a url users are sent to as part of a complex user journey: Here\'s the url in question: https://mysite.org/user/login/idp_sign_up%3FpostbackUrl%3Dhttps%3A//hub.somesite.org.uk%3A443/govuk/register-signin%3FrequestId%3D_ae0b3a45-c878-4033-96f2-2b2428090711 I\'m parsing the necessary bits out of this fine, if user is signed in. My problem occurs when a user isn\'t signed in, because they\'re redirected to this url: https://mysite.org/user/login?destination=/idp_sign_up%3FpostbackUrl%3Dhttps%3A//hub.somesite.org.uk%3A443/register-signin%3FrequestId%3D_ae0b3a45-c878-4033-96f2-2b2428090711 Then they end up at this url, without the crucial %3FrequestId%3D_ae0b3a45-c878-4033-96f2-2b2428090711 fragment: https://mysite.org/user/login/idp_sign_up%3FpostbackUrl%3Dhttps%3A//hub.somesite.org.uk%3A443/register-signin How to I ensure I don\'t lose that fragment if a user isn\'t logged in? Is there a relevant hook I can use before the https://mysite.org/user/login?destination process begins?