textFieldAlert.dart

showDialog(
  context: context,
  builder: (BuildContext context) {
    return AlertDialog(
           title: Text("Title", textAlign: TextAlign.center),
           content: TextField(
             decoration: InputDecoration(hintText: "Jhon Doe", labelText: 'Enter your username'),
           ),
           actions: <Widget>[
               FlatButton(
                child: Text("Agree"),
                onPressed: () {
                Navigator.of(context).pop();
                },
               ),
               FlatButton(
                child: Text("Close"),
                   onPressed: () {
                     Navigator.of(context).pop();
                  },
                ),
               ],
            );

            },
        );