How to add the "audience" attribute to the request body of the token request with FeignClient and OAuth2 in Spring Boot (Kotlin)?

Question

I am currently using Feign, a declarative REST client, with OAuth2 in my Spring Boot (Kotlin) application. To specify the client provider and registration, I use the application.yml properties file with the grant_type set to client_credentials and the token-uri endpoint set to https://url/oauth/token.

Error: A 403 (Forbidden) error message stating that No audience parameter was provided... because the audience attribute is missing from the request body.

Is it possible to add the audience attribute with a URL value to the token request using FeignClient and OAuth2 in Spring Boot (Kotlin) via configuration properties or do I need to create a custom bean?

I have tried adding the audience attribute using custom parameters in the header and query params, but this did not work. https://docs.spring.io/spring-security/reference/servlet/oauth2/index.html#oauth2-client-customize-request-parameters

I need to add the audience attribute with a value to the request body of the token request.

To enable OAuth2 support, I am using this: https://docs.spring.io/spring-cloud-openfeign/docs/current/reference/html/#oauth2-support.

To add the audience attribute to the request body, I need to provide it in the following format:

POST call to /oauth/token:

{
    "grant_type": "client_credentials",
    "client_id": "id",
    "client_secret": "secret",
    "audience": "https://url"
}

The response should be in the following format:

{
    "access_token": "token",
    "expires_in": 86400,
    "token_type": "Bearer"
}

Good to mention that I'm not the owner of the Authentication (0Auth) server and therefore cannot provide a default audience or make any changes.