You can use boxShadow for this. Try this code.
Scaffold( body: Center( child: ElevatedButton( child: const Text('Press'), onPressed: () { showModalBottomSheet( barrierColor: Colors.transparent, context: context, builder: (context) { return Container( height: 300, decoration: BoxDecoration( color: Colors.white, borderRadius: BorderRadius.vertical(top: Radius.circular(30)), boxShadow: [BoxShadow(color: Colors.grey, blurRadius: 10.0)], ), ); }, ); }, ), ), );

Here is the code for your problem. You just have to take a simple container with a border radius in boxdecoration.
new Container( alignment: Alignment.center, decoration: BoxDecoration( borderRadius: BorderRadius.all(Radius.circular(15.0)), color: Colors.blue, ),
child: Row( mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[ Padding( padding: const EdgeInsets.all(10.0), child: new Text( "Next", style: new TextStyle( fontWeight: FontWeight.w500, color: Colors.white, fontSize: 15.0, ), ), ), ], ), ),