At the time of writing the Kinde Ruby SDK doesn't quite work (it is auto generated) when trying to get a management token for use with the Kinde Management API.

A simple way to do this is to use an HTTP library of your choosing - I like the HTTP gem

Assuming you have setup the secrets using the credentials files:

EDITOR="code --wait" bin/rails credentials:edit --environment=development

With the secrets:

kinde:
  client_id: "CLIENT_ID"
  client_secret: "CLIENT_SECRET"
  domain: "https://YOUR_DOMAIN"

Create a new service and use the code below:

module Kinde
  module Services
    class GetManagementToken

      def perform
        raw_token = get_token
        return { success: false, errors: ["Unable to get token"] } if raw_token.blank?

        parsed_token = parse_token(raw_token)
        return { success: false, errors: ["Unable to parse token"] } if parsed_token.blank?

        return { success: true, data: parsed_token }
      end

      private

        def get_token
          HTTP.post("#{domain_url}/oauth2/token",
            form: {
              audience: "https://api.kinde-auth.app",
              client_id: Rails.application.credentials.dig(:kinde, :client_id),
              client_secret: Rails.application.credentials.dig(:kinde, :client_secret),
              grant_type: "client_credentials",
            },
          ).to_s
        end

        def parse_token(raw_token)
          begin
            JSON.parse(raw_token, symbolize_names: true)
          rescue => exception
            nil        
          end
        end

        def domain_url
          Rails.application.credentials.dig(:kinde, :domain)
        end
    end
  end
end

Then just call the service:

Kinde::Services::GetManagementToken.new.perform