sp-dev-fx-webparts/samples/react-provision-assets/config/serve.json

{
  "port": 4321,
  "initialPage": "https://localhost:5432/workbench",
  "https": true,
  "api": {
    "port": 5432,
    "entryPath": "node_modules/@microsoft/sp-webpart-workbench/lib/api/"
  }
}