flutter custom shape button in bottom navbar

I have this design of a bottom nav bar(shown in photo) and I can't figure out how to make the custom shape button, here is my bottom navbar code

Scaffold(
      appBar: AppBar(
        title: Text("Reply demo"),
      ),
      bottomNavigationBar: BottomAppBar(
        elevation: 0,
        color: Color(0xff344955),
        child: Container(
          padding: EdgeInsets.symmetric(horizontal: 10.0),
          height: 56.0,
          child: Row(children: <Widget>[
            IconButton(
              onPressed: showMenu,
              icon: Icon(Icons.menu),
              color: Colors.white,
            ),
            Spacer(),
            IconButton(
              onPressed: () {},
              icon: Icon(Icons.add),
              color: Colors.white,
            ),
            IconButton(
              onPressed: () {},
              icon: Icon(Icons.home),
              color: Colors.white,
            )
          ]),
        ),
      ),