Rest API call in custom policy in Azure AD B2C - azure

Question

I am new and have tried to build custom policy for sign-in by invoking a REST API call to legacy.
The API expects below parameters as input:
client_id=fixed value, client_secret=fixed value, grant_type=fixed value, scope=fixed value, username=variable, password=variable
The above parameters are not for Azure valued but some fixed values that need to be sent in the request.
I have added few portions in the extensions custom policy but seems the request body is not forming correctly and I am getting the error The claims exchange User Migration Via Legacy specified in step '1' returned HTTP error response with Code Bad Request and Reason 'Bad Request'.
What am I doing wrong here? Please help.
Added my portion :
<BuildingBlocks>
<ClaimsSchema>
<ClaimType Id="RequestBody">
<DisplayName>Request body</DisplayName>
<DataType>string</DataType>
<UserHelpText>RequestBody</UserHelpText>
</ClaimType>
</ClaimsSchema>
<ClaimsTransformations>
<ClaimsTransformation Id="GenerateRequestBody" TransformationMethod="GenerateJson">
<InputClaims>
<InputClaim ClaimTypeReferenceId="email" TransformationClaimType="username" />
<InputClaim ClaimTypeReferenceId="password" TransformationClaimType="password" />
</InputClaims>
<InputParameters>
<InputParameter Id="client_id" DataType="string" Value="client" />
<InputParameter Id="client_secret" DataType="string" Value="sec" />
<InputParameter Id="grant_type" DataType="string" Value="grant" />
<InputParameter Id="scope" DataType="string" Value="scope" />
</InputParameters>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="RequestBody" TransformationClaimType="outputClaim" />
</OutputClaims>
</ClaimsTransformation>
</ClaimsTransformations>
<ClaimsProviders>
<ClaimsProvider>
<DisplayName>REST API to communicate with Legacy IdP</DisplayName>
<TechnicalProfiles>
<TechnicalProfile Id="UserMigrationViaLegacyIdp">
<DisplayName>REST API call to communicate with Legacy IdP</DisplayName>
<Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.RestfulProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
<Metadata>
<Item Key="ServiceUrl">https://</Item>
<Item Key="AuthenticationType">None</Item>
<Item Key="SendClaimsIn">Body</Item>
<Item Key="AllowInsecureAuthInProduction">true</Item>
<Item Key="ClaimUsedForRequestPayload">RequestBody</Item>
</Metadata>
<InputClaimsTransformations>
<InputClaimsTransformation ReferenceId="GenerateRequestBody" />
</InputClaimsTransformations>
<InputClaims>
<InputClaim ClaimTypeReferenceId="RequestBody" />
</InputClaims>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="tokenSuccess" DefaultValue="false" />
<OutputClaim ClaimTypeReferenceId="migrationRequired" />
</OutputClaims>
<UseTechnicalProfileForSessionManagement ReferenceId="SM-Noop" />
</TechnicalProfile>
</TechnicalProfiles>
</ClaimsProvider>

Answer 1

Can you share the actual JSON created by the claim transformation? Is it well formatted? You can make troubleshooting easy by displaying the output of <InputClaimsTransformation ReferenceId="GenerateRequestBody" /> before you actually use it as input claim inside the REST API technical profile. This will reveal what's going on with your JSON format.
To do that, first create a self asserted technical profile and add <InputClaimsTransformation ReferenceId="GenerateRequestBody" /> as an output claim transformation to it. Your output claim should be the claim which you will use later in next step in the journey.
In the user journey, add a step that calls the self asserted technical profile. Add next step with App Insights to write the output claim to App Insights( https://learn.microsoft.com/en-us/azure/active-directory-b2c/analytics-with-application-insights)
This will document claim value that REST API is receiving.