example.dart

import 'dart:convert';
import 'package:convert/convert.dart';
import 'package:crypto/crypto.dart';
import 'package:http/http.dart' as http;

encode() async {
  var secret = 'aefij3ldaase_ase23fdAdwjnA2123fFa';
  var body = 'apikey=' +
      'dgsdrij234fsdfgkhr' +
      '&nonce=' +
      DateTime.now().millisecondsSinceEpoch.toString() +
      '&offer_hash=Agq1Bpw7oX9&margin=50';

  var hmacEncoder = new Hmac(sha256, secret); // HMAC-SHA256
  var seal = hmacEncoder.convert(body);

  var headers = {
    'Content-type': 'application/json;charset=UTF-8',
    'Accept': 'application/json',
  };

  var response = await http.post(
      "https://www.paxful.com/api/offer/list&apiseal=$seal",
      body: body,
      headers: headers);
}