How to set rounded border to a MaterialButton on Flutter?

If you need to use MaterialButton() - You need to Warp the button with Material Widget, to get the desired behavior.

    Widget _showNeedHelpButton() {
    return Padding(
      padding: EdgeInsets.fromLTRB(0.0, 5.0, 0.0, 0.0),
      child: Material(  //Wrap with Material
        shape: RoundedRectangleBorder(borderRadius:BorderRadius.circular(22.0) ),
        elevation: 18.0,
        color: Color(0xFF801E48),
        clipBehavior: Clip.antiAlias, // Add This
        child: MaterialButton(
          minWidth: 200.0,
          height: 35,
          color: Color(0xFF801E48),
          child: new Text('Preciso de ajuda',
              style: new TextStyle(fontSize: 16.0, color: Colors.white)),
          onPressed: () {
//          setState(() {
//            _isNeedHelp = true;
//          });
          },
        ),
      ),
    );
  }

Output:

UPDATE:

  minWidth: 200.0,
  height: 95,