If this is solely for reporting these events to GA4, you can send them directly to the MP endpoint.
If you also want to use these events for Google Ads, the best approach is to send webhooks to the server container, where they will be received and processed by the Data Client. In this case, you can use the freeform JSON payload format.
However, as this is not an online conversion, you will also need to use a different type of Google Ads tag, namely the Google Ads offline conversion tag.