To customize Caddy, you can add routes to the Caddy routes for processing a request. 

For example, you can add a header with the following JSON snippet:

{
    "handle": [
      {
        "handler": "headers",
        "request": {
          "set": {
            "X-Request-Start": [
              "{time.now.unix_ms}"
            ]
          }
        }
      }
    ]
  }

The easiest way to generate these JSON snippets is to write a Caddyfile locally then run caddy adapt to convert it to JSON. Then you can grab the route handler snippet and copy and paste it into the route configuration in your Hatchbox app.