Send custom query param to identity provider (screen_hint)
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Background
I've integrated a third party OIDC IdP with FusionAuth. This IdP supports a non-standard screen_hint query param on their OAuth authorization endpoint. If the IdP receives screen_hint=register at their authorization endpoint then they will render their registration page by default instead of their sign in page.
End goal
I would like to be able to send a user to FusionAuth with both the idp_hint and screen_hint params set and have the screen_hint parameter be passed on to the IdP's authorization endpoint. This will allow me to render a "Create account" button in my app that will send the user straight to the IdP's registration page.
What I've tried
I've tried digging through FusionAuth docs, poking around our custom freemarker templates, watching the network tab, and searching through this forum but have come up short. An adjustment to our custom freemarker theme templates seemed like the best bet, but it appears the IdP authorization request is constructed on the server so no amount of tinkering with client-side logic can add a custom parameter. I also want to make sure that this parameter is compatible with idp_hint, and I'm not confident any client-side theme logic would be compatible with that since idp_hint is designed to skip the UI.
Any solutions or workarounds would be appreciated!