Azure AD B2C custom policy - send query string parameter to 3rd party

Question

I am working on a B2C case that involves integration with a 3rd party identity provider using OpenIdConnect.

This third party expects a specific query string parameter to be sent one or more times:

example.com/oidc/authorize=?...& login_hint =ABCvalue& login_hint =XYZvalue

I have an OpenIdConnect Technical Profile with the required input claims:

<TechnicalProfile Id="Example-Swe">
  <DisplayName>Example Sverige</DisplayName>
  <Description>Login with Example</Description>
  <Protocol Name="OpenIdConnect"/>
  <OutputTokenFormat>JWT</OutputTokenFormat>
  <Metadata>
    <Item Key="client_id">preprod.Example</Item>
    <Item Key="METADATA">https://example.com/oidc/.well-known/openid-configuration</Item>
    <Item Key="response_types">code</Item>
    <Item Key="scope">openid phone profile example.national_id offline_access</Item>
    <Item Key="response_mode">query</Item>
    <Item Key="HttpBinding">POST</Item>
    <Item Key="UsePolicyInRedirectUri">false</Item>
  </Metadata>
  <CryptographicKeys>
    <Key Id="client_secret" StorageReferenceId="B2C_1A_ExampleSecretNO1"/>
  </CryptographicKeys>
  <InputClaims>
    <InputClaim ClaimTypeReferenceId="acr_values" DefaultValue="urn:example:oidc:method:sbid"/>
    <InputClaim ClaimTypeReferenceId="example_profile" DefaultValue="example-id" />
    <InputClaim ClaimTypeReferenceId="ui_locales" DefaultValue="sv" />
    <InputClaim ClaimTypeReferenceId="claimABC" PartnerClaimType="login_hint" />
    <InputClaim ClaimTypeReferenceId="claimXYZ" PartnerClaimType="login_hint" />
  </InputClaims>
  <OutputClaims>
    <OutputClaim ClaimTypeReferenceId="access_token" PartnerClaimType="{oauth2:access_token}" />
    <OutputClaim ClaimTypeReferenceId="issuerUserId" PartnerClaimType="sub" />
    <OutputClaim ClaimTypeReferenceId="authenticationSource" DefaultValue="SigBidSwe" />
    <OutputClaim ClaimTypeReferenceId="identityProvider" DefaultValue="Example-BID-Swe" />
    <OutputClaim ClaimTypeReferenceId="countryCode" DefaultValue="SE" AlwaysUseDefaultValue="true" />

  </OutputClaims>
  <UseTechnicalProfileForSessionManagement ReferenceId="SM-AAD"/>
</TechnicalProfile>

but having two claim with the same PartnerClaimType does not work. B2C throws an Exception.

Is it possible to achieve this somehow?

Answer 1

Can't be done today with Azure B2C custom policies and also the third party is not following the spec.

Duplicate query parameters violates the OAUTH specification

5. Parameters

OAuth Protocol Parameter names and values are case sensitive. Each OAuth Protocol Parameters MUST NOT appear more than once per request, and are REQUIRED unless otherwise noted.

https://oauth.net/core/1.0a/