sp-dev-fx-webparts/samples/react-pages-hierarchy/config/serve.json

{
  "$schema": "https://developer.microsoft.com/json-schemas/core-build/serve.schema.json",
  "port": 4321,
  "https": true,
  "initialPage": "https://enter-your-SharePoint-site/_layouts/workbench.aspx",
  "api": {
    "port": 5432,
    "entryPath": "node_modules/@microsoft/sp-webpart-workbench/lib/api/"
  }
}