mempool/frontend/proxy.conf.json

{
  "/api": {
    "target": "http://localhost:8999/",
    "secure": false
  },
  "/ws": {
    "target": "http://localhost:8999/",
    "secure": false,
    "ws": true
  },
  "/electrs": {
    "target": "https://www.blockstream.info/testnet/api/",
    "secure": false,
    "pathRewrite": {
      "^/electrs": ""
    }
  }
}