How can I read a value from header in Azure API Management and pass it to a path param in keycloak realm?

Read value from header in api policy and pass it to api path param in azure policy

Hi I have a requirement where I use keycloak to authenticate my urls. Below is my policy

<policies>
	<inbound>
		<base />
		<validate-jwt header-name="Authorization" failed-validation-httpcode="401" failed-validation-error-message="Authorization Failed. Check Bearer Token">
			<openid-config url="https://keycloak.digit.i/realms/{realmName}/.well-known/openid-configuration" />
		</validate-jwt>
	</inbound>
	<backend>
		<base />
	</backend>
	<outbound>
		<base />
	</outbound>
	<on-error>
		<base />
	</on-error>
</policies>

Now i have a requirement like I have to read a value from header and pass it to path param of the openid-config url . Below is something i want to achieve

<policies>
	<inbound>
		<base />
                <set-variable name="HostName" value="@{
            string [] HostNameHeader;
            
            context.Request.Headers.TryGetValue("Host", out HostNameHeader);    

            return HostNameHeader[0];
        }" />
		<validate-jwt header-name="Authorization" failed-validation-httpcode="401" failed-validation-error-message="Authorization Failed. Check Bearer Token">
			<openid-config url="https://keycloak.digit.i/realms/{HostName}/.well-known/openid-configuration" />
		</validate-jwt>
	</inbound>
	<backend>
		<base />
	</backend>
	<outbound>
		<base />
	</outbound>
	<on-error>
		<base />
	</on-error>
</policies>

Kindly guide as how I can achieve it

I have reproduced the reported issue in my environment and got the below results-

Here I am using Azure AD for openid-config URL (https://login.microsoftonline.com/{aad-tenant}/.well-known/openid-configuration for v1 endpoint and https://login.microsoftonline.com/{aad-tenant}/v2.0/.well-known/openid-configuration for v2 endpoint) where aad-tenant is my Azure AD tenant-id.

I am reading tenant-id from request header and passing it to openid-config URL .

Policy-

<policies>
<inbound>
<base  />
<set-variable  name="aad-tenant"  value="@(context.Request.Headers.GetValueOrDefault("tenantId", ""))"  />
<validate-jwt  header-name="Authorization"  failed-validation-httpcode="401"  failed-validation-error-message="Unauthorized. Access token is missing or invalid.">
<openid-config  url="@($"https://login.microsoftonline.com/{(string)context.Variables["aad-tenant"]}/.well-known/openid-configuration")"  />
</validate-jwt>
</inbound>
<backend>
<base  />
</backend>
<outbound>
<base  />
</outbound>
<on-error>
<base  />
</on-error>
</policies>

Test Result in API Management service-