Hosting the id_token_hint well-known endpoint in Azure AD B2C itself

Username_ProofUp_MLB2C.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<TrustFrameworkPolicy
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                      xmlns:xsd="http://www.w3.org/2001/XMLSchema"
                      xmlns="http://schemas.microsoft.com/online/cpim/schemas/2013/06"
                      PolicySchemaVersion="0.3.0.0"
                      TenantId="tenant.onmicrosoft.com"
                      PolicyId="B2C_1A_Username_ProofUp_MLB2C"
                      PublicPolicyUri="http://tenant.onmicrosoft.com/B2C_1A_Username_ProofUp_MLB2C"
                      DeploymentMode="Development"
                      UserJourneyRecorderEndpoint="urn:journeyrecorder:applicationinsights">

	<BasePolicy>
		<TenantId>tenant.onmicrosoft.com</TenantId>
		<PolicyId>B2C_1A_TrustFrameworkExtensionsMFA</PolicyId>
	</BasePolicy>

	<BuildingBlocks>
		<ClaimsSchema>
						
			<ClaimType Id="mail">
				<DisplayName>Email Address</DisplayName>
				<DataType>string</DataType>
				<AdminHelpText>Email addresses of the user.</AdminHelpText>
				<UserHelpText>Your email addresses.</UserHelpText>
				<UserInputType>TextBox</UserInputType>
			</ClaimType>
			
			<ClaimType Id="strongAuthenticationEmailAddress">
				<DisplayName>Email Address</DisplayName>
				<DataType>string</DataType>
				<AdminHelpText>Email address that the user can use for strong authentication.</AdminHelpText>
				<UserHelpText>Email address to use for strong authentication.</UserHelpText>
				<UserInputType>TextBox</UserInputType>
			</ClaimType>
			
			<ClaimType Id="signInNamesInfo.emailAddress">
				<DisplayName>Email Address</DisplayName>
				<DataType>string</DataType>
				<AdminHelpText>Email address that the user can use to sign in.</AdminHelpText>
				<UserHelpText>Email address to use for signing in.</UserHelpText>
				<UserInputType>TextBox</UserInputType>
			</ClaimType>
			
			<ClaimType Id="emails">
				<DisplayName>Email Addresses</DisplayName>
				<DataType>stringCollection</DataType>
				<AdminHelpText>Email addresses of the user.</AdminHelpText>
				<UserHelpText>Your email addresses.</UserHelpText>
			</ClaimType>			
			
		</ClaimsSchema>
		
		<ClaimsTransformations>
			
			<ClaimsTransformation Id="CreateSubjectClaimFromObjectID" TransformationMethod="CreateStringClaim">
				<InputParameters>
					<InputParameter Id="value" DataType="string" Value="Not supported currently. Use oid claim."/>
				</InputParameters>
				<OutputClaims>
					<OutputClaim ClaimTypeReferenceId="sub" TransformationClaimType="createdClaim"/>
				</OutputClaims>
			</ClaimsTransformation>
			
			<ClaimsTransformation Id="CreateEmailsFromOtherMailsAndSignInNamesInfo" TransformationMethod="AddItemToStringCollection">
				<InputClaims>
					<InputClaim ClaimTypeReferenceId="signInNamesInfo.emailAddress" TransformationClaimType="item"/>
					<InputClaim ClaimTypeReferenceId="otherMails" TransformationClaimType="collection"/>
				</InputClaims>
				<OutputClaims>
					<OutputClaim ClaimTypeReferenceId="emails" TransformationClaimType="collection"/>
				</OutputClaims>
			</ClaimsTransformation>

			<ClaimsTransformation Id="AddStrongAuthenticationEmailToEmails" TransformationMethod="AddItemToStringCollection">
				<InputClaims>
					<InputClaim ClaimTypeReferenceId="strongAuthenticationEmailAddress" TransformationClaimType="item"/>
					<InputClaim ClaimTypeReferenceId="emails" TransformationClaimType="collection"/>
				</InputClaims>
				<OutputClaims>
					<OutputClaim ClaimTypeReferenceId="emails" TransformationClaimType="collection"/>
				</OutputClaims>
			</ClaimsTransformation>			
			
		</ClaimsTransformations>
	</BuildingBlocks>

	<ClaimsProviders>		
	
		<ClaimsProvider>
			<DisplayName>Local Account Username</DisplayName>
			<TechnicalProfiles>

				<TechnicalProfile Id="AAD-UserReadUsingUserName">
					<Metadata>
						<Item Key="Operation">Read</Item>
						<Item Key="RaiseErrorIfClaimsPrincipalDoesNotExist">true</Item>
					</Metadata>					
					<InputClaims>
						<InputClaim ClaimTypeReferenceId="signInName" PartnerClaimType="signInNames.userName"/>						
					</InputClaims>					
					<OutputClaims>
						<OutputClaim ClaimTypeReferenceId="objectId"/>
						<OutputClaim ClaimTypeReferenceId="displayName"/>
						<OutputClaim ClaimTypeReferenceId="givenName"/>
						<OutputClaim ClaimTypeReferenceId="surname"/>						
						<OutputClaim ClaimTypeReferenceId="signInName"/>
						<OutputClaim ClaimTypeReferenceId="mail"/>
						<OutputClaim ClaimTypeReferenceId="accountEnabled"/>
						<OutputClaim ClaimTypeReferenceId="authenticationSource" DefaultValue="localAccountAuthentication"/>
						<OutputClaim ClaimTypeReferenceId="strongAuthenticationEmailAddress"/>						
					</OutputClaims>
					<OutputClaimsTransformations>
						<OutputClaimsTransformation ReferenceId="CreateSubjectClaimFromObjectID"/>						
					</OutputClaimsTransformations>
					<IncludeTechnicalProfile ReferenceId="AAD-ReadCommon"/>
				</TechnicalProfile>
				
				<TechnicalProfile Id="AAD-ReadCommon">
					<Metadata>
						<Item Key="Operation">Read</Item>
						<Item Key="RaiseErrorIfClaimsPrincipalDoesNotExist">true</Item>
					</Metadata>
					<OutputClaims>
						<OutputClaim ClaimTypeReferenceId="userPrincipalName"/>
						<OutputClaim ClaimTypeReferenceId="displayName"/>						
						<OutputClaim ClaimTypeReferenceId="otherMails"/>
						<OutputClaim ClaimTypeReferenceId="strongAuthenticationEmailAddress" PartnerClaimType="signInNames.emailAddress"/>
					</OutputClaims>
					<OutputClaimsTransformations>
						<OutputClaimsTransformation ReferenceId="CreateEmailsFromOtherMailsAndSignInNamesInfo"/>
						<OutputClaimsTransformation ReferenceId="AddStrongAuthenticationEmailToEmails"/>
					</OutputClaimsTransformations>
					<IncludeTechnicalProfile ReferenceId="AAD-Common"/>
				</TechnicalProfile>
				
				<TechnicalProfile Id="IdTokenHint_Asymmetric_ExtractClaims">
					<DisplayName>My ID Token Hint Asymmetric Technical Profile</DisplayName>
					<Protocol Name="None"/>
					<Metadata>
						<!-- Replace with your endpoint location -->						
						<Item Key="METADATA">https://tenant.b2clogin.com/tenant.onmicrosoft.com/v2.0/.well-known/openid-configuration?p=B2C_1A_SIGNUP_SIGNIN_MLB2C</Item>
						<Item Key="IdTokenAudience">7bd30dbe...1d14760</Item>
						<Item Key="issuer">http://localhost:57408/</Item>
					</Metadata>
					<OutputClaims>
						<OutputClaim ClaimTypeReferenceId="signInName" PartnerClaimType="signInNames.userName"/>						
					</OutputClaims>
				</TechnicalProfile>
			</TechnicalProfiles>
		</ClaimsProvider>	
		
	</ClaimsProviders>

	<UserJourneys>
	
		<UserJourney Id="ProofUp_MagicLink_B2C">
			<OrchestrationSteps>
			
				<OrchestrationStep Order="1" Type="GetClaims" CpimIssuerTechnicalProfileReferenceId="IdTokenHint_Asymmetric_ExtractClaims"/>
				
				<OrchestrationStep Order="2" Type="ClaimsExchange">
					<ClaimsExchanges>
						<ClaimsExchange Id="Read-WithUsername" TechnicalProfileReferenceId="AAD-UserReadUsingUserName"/>
					</ClaimsExchanges>
				</OrchestrationStep>				

				<OrchestrationStep Order="3" Type="SendClaims" CpimIssuerTechnicalProfileReferenceId="JwtIssuer"/>

			</OrchestrationSteps>
			<ClientDefinition ReferenceId="DefaultWeb"/>
		</UserJourney>
	</UserJourneys>
  
	<RelyingParty>
		<DefaultUserJourney ReferenceId="ProofUp_MagicLink_B2C"/>
		<UserJourneyBehaviors>
			<JourneyInsights TelemetryEngine="ApplicationInsights" InstrumentationKey="xyz" DeveloperMode="true" ClientEnabled="false" ServerEnabled="true" TelemetryVersion="1.0.0"/>			
		</UserJourneyBehaviors>
		<TechnicalProfile Id="PolicyProfile">
			<DisplayName>PolicyProfile</DisplayName>
			<Protocol Name="OpenIdConnect"/>
			<InputClaims>
				<InputClaim ClaimTypeReferenceId="signInName" PartnerClaimType="signInNames.userName"/>
			</InputClaims>
			<OutputClaims>
				<OutputClaim ClaimTypeReferenceId="displayName"/>
				<OutputClaim ClaimTypeReferenceId="givenName"/>
				<OutputClaim ClaimTypeReferenceId="surname"/>				
				<OutputClaim ClaimTypeReferenceId="signInName"/>
				<OutputClaim ClaimTypeReferenceId="objectId" PartnerClaimType="sub"/>
				<OutputClaim ClaimTypeReferenceId="identityProvider" DefaultValue="Local"/>
				<OutputClaim ClaimTypeReferenceId="tenantId" AlwaysUseDefaultValue="true" DefaultValue="{Policy:TenantObjectId}"/>
			</OutputClaims>
			<SubjectNamingInfo ClaimType="sub"/>
		</TechnicalProfile>
	</RelyingParty>
</TrustFrameworkPolicy>

https://medium.com/the-new-control-plane/hosting-the-id-token-hint-well-known-endpoint-in-azure-ad-b2c-itself-1c27be147613