Adding a Scrollbar to a CustomScollView

I've been trying to figure out how to add a Scrollbar Widget to a CustomScrollView. One would usually just have to wrap a ListView or ScrollView with a Scrollbar Widget, pass in the same ScrollController and boom, it's rendered.

Here is a minimal code block.

CustomScrollView(
            slivers: [
              SliverAppBar(
                title: Hero(
                  tag: 'title',
                  child: RichText(
                    text: TextSpan(
                      children: [
                        TextSpan(
                          text: 'Not',
                          style: TextStyle( fontSize: 20, fontWeight: FontWeight.w500),
                        ),
                        TextSpan(
                          text: '3s',
                          style: TextStyle(fontSize: 20, fontWeight: FontWeight.w500),
                        )
                      ],
                    ),
                  ),
                ),
              ),
              SliverList(
                delegate: SliverChildBuilderDelegate(
                  (context, index) {
                    return Container(); })));