Extract login_hint for sign up custom policy in Azure to pre-fill email

We have an angular app and have created a custom policy for signing up users. When using a regular user flow the email gets automatically pre-filled. This doesn't seem to be the case with the custom policy. So i tried the following...

<ClaimType Id="login_hint">
<DisplayName>Login Hint</DisplayName>
<DataType>string</DataType>
<DefaultPartnerClaimTypes>
<Protocol Name="OAuth2" PartnerClaimType="login_hint" />
</DefaultPartnerClaimTypes>
<UserHelpText>Login hint for pre-filling email.</UserHelpText>
</ClaimType>
...
<ClaimsTransformation Id="ExtractLoginHintToEmail" TransformationMethod="FormatStringClaim">
<InputClaims>
<InputClaim ClaimTypeReferenceId="login_hint" TransformationClaimType="inputClaim" />
</InputClaims>
<InputParameters>
<InputParameter Id="stringFormat" DataType="string" Value="{0}" />
</InputParameters>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="email" TransformationClaimType="outputClaim" />
</OutputClaims>
</ClaimsTransformation>
...
<ClaimsProvider>
<DisplayName>Email Verification</DisplayName>
<TechnicalProfiles>
<!--Email verification only-->
<TechnicalProfile Id="EmailVerification">
<DisplayName>Initiate Email Address Verification For Local Account</DisplayName>
<Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.SelfAssertedAttributeProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
<Metadata>
<Item Key="ContentDefinitionReferenceId">api.localaccount.emailVerification</Item>
<Item Key="language.button_continue">Continue</Item>
</Metadata>
<InputClaimsTransformations>
<!-- Use the login_hint parameter to populate the email claim -->
<InputClaimsTransformation ReferenceId="ExtractLoginHintToEmail" />
</InputClaimsTransformations>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="Verified.Email" Required="true" />
</OutputClaims>
</TechnicalProfile>
</TechnicalProfiles>
</ClaimsProvider>


But when running the policy i get The page cannot be displayed because an internal server error has occurred. . So something is broken.. If i remove the ClaimType, ClaimsTransformation and this line and the policy runs just fine

If anyone has any idea, I would greatly appreciate it.

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