You can simply use stack to position it :)

body: Stack(
    children: [
      Positioned(
        top: -240,
        child: Container(
          height: 500,
          width: MediaQuery.sizeOf(context).width ,
          decoration: BoxDecoration(
            color: AppColors.white,
            borderRadius: BorderRadius.all(Radius.circular(1000))
          ),
        ),
      )
    ],
  ),