Calling payment is easy but to make it work isn't not so easy:
payWithWeChat(
appId: result['appid'],
partnerId: result['partnerid'],
prepayId: result['prepayid'],
packageValue: result['package'],
nonceStr: result['noncestr'],
timeStamp: result['timestamp'],
sign: result['sign'],
);
Take a look at payment document for help.