From your Gitlab admin area, add a new application with the following settings:
|Trusted / Confidential
After creating the application, Gitlab will give you both the Client ID and Client Secret, which they respectively name Application ID and Application Secret.
You may need to check "Allow requests to the local network from web hooks and services" in Admin / Settings / Network / Outbound requests for webhooks to be sent to Meli.
Now, you can update your