broadcast.dart

// pubspec.yaml
// dev_dependencies:
//   dio: 3.0.9

// Blocypher Docs: https://www.blockcypher.com/dev/bitcoin/?javascript#push-raw-transaction-endpoint
// https://github.com/flutterchina/dio#super-simple-to-use
import 'package:dio/dio.dart';

void broadcastTx(String hex) async {
  try {
    Response response = await Dio().post("https://api.blockcypher.com/v1/bcy/test/txs/push?token=<your-secret-blockypher-token-here>", data: { tx: hex } );
    print(response);
  } catch (e) {
    print(e);
  }
}

main() {
  broadcastTx("your-tx-hex here: 01000000011935b41d12936df99d322....");
}