Validating Authorization token in incoming requests with Azure API management and third party Authorization Server - azure

Question

I have to implement OAuth 2.0 in my project with a third party Authorization Server. Both my client and server are already registered on the AS. I have created an API management instance on Azure and imported the swagger APIs. I want every incoming request to be validated against my AS, so I just need to redirect the request to https://my-as.com/as/introspect.oauth2, and have the token validated. If the token is valid then let it proceed or else send 401. I was trying to implement this using "inbound processing" and referred to the following doc: https://learn.microsoft.com/en-us/azure/api-management/api-management-howto-protect-backend-with-aad#configure-a-jwt-validation-policy-to-pre-authorize-requests.
The only problem is that instead of Azure AD I'm using a third party AS. I tried replacing the URL in sample XML code with my URL, but its not working.
How do I redirect requests to the Authorization Server for validating access token?

Answer 1

Adding following inbound policy worked:
<inbound>
<!-- Extract Token from Authorization header parameter -->
<set-variable name="token" value="#(context.Request.Headers.GetValueOrDefault("Authorization","scheme param").Split(' ').Last())" />
<!-- Send request to Token Server to validate token (see RFC 7662) -->
<send-request mode="new" response-variable-name="tokenstate" timeout="20" ignore-error="true">
<set-url>https://my-as.com/as/introspect.oauth2</set-url>
<set-method>POST</set-method>
<set-header name="Content-Type" exists-action="override">
<value>application/x-www-form-urlencoded</value>
</set-header>
<set-body>#($"grant_type=urn:pingidentity.com:oauth2:grant_type:validate_bearer&client_id=UoM&client_secret=somesecret&token={(string)context.Variables["token"]}")</set-body>
</send-request>
<choose>
<!-- Check active property in response -->
<when condition="#((bool)((IResponse)context.Variables["tokenstate"]).Body.As<JObject>()["active"] == false)">
<!-- Return 401 Unauthorized with http-problem payload -->
<return-response response-variable-name="existing response variable">
<set-status code="401" reason="Unauthorized" />
<set-header name="WWW-Authenticate" exists-action="override">
<value>Bearer error="invalid_token"</value>
</set-header>
</return-response>
</when>
</choose>
<base />
</inbound>