Cognito does not pass 'login_hint' to Federated SAML Identity Provider

On my initial request to https://(my domain).auth.(region).amazoncognito.com/oauth2/authorize I can see that the login_hint parameter is present on the query string. That value is unfortunately not present on the redirect to the SAML2 endpoint.

The result of this is that users must enter their e-mail address first on our site, and then a second time at their identity provider.

How do I specify this value on the /authorize request in such a way that it will be passed through?