got
    .post("domain.api", {
      hooks: {
        beforeRequest: [
          (options) => {
            const newJsonBody = {
              ...options.json,
              new_field: "new_field",
            };

            // Original: not working
            options.body = JSON.stringify(newJsonBody);

            // Fixed
            // options.body = Buffer.from(JSON.stringify(newJsonBody));

            options.headers["content-length"] = options.body.length.toString();
          },
        ],
      },
      json: {
        city: "Hà Nội", // Unicode characters
      },
    })