How can I pass client_id and client_secret in the body of the token exchange request using the react-native-app-auth package?

Question

I am trying to use the otherwise excellent react-native-app-auth to handle the OAuth part of an app I'm working on.

The server I am contacting expects to have the client_id and client_secret included in the body of the token exchange POST request, but it is not being included and therefore results always in a 401 unauthorised response.

I have tried many different config options as specified in the docs, such as passing the client_id and secret in additionalParameters and customHeaders.

// Passing this to the authorize method.
const config = {
            issuer: 'https://example.dev',
            clientId: client_id,
            clientSecret: client_secret
            redirectUrl: 'myapp://auth_success',
            scopes: scopes,
            serviceConfiguration: {
                tokenEndpoint: 'https://example.dev/access-token',
                authorizationEndpoint: `https://example.dev?`,
            }

Expected is a 200 response with a fresh auth and refresh token. Instead I get back "parameter "client_secret" and "client_id" is missing.