If the scroll box was on top of a uniform colour, I could obviously place a gradient on top of it and it would fake the effect. However, this widget has an animated background, so that’s not possible.
I just stumbled upon a similar issue, so thought I’d share my findings in case it helps others who are trying to figure out this as well.
Short answer: wrap your scroll box with a retainer box.
From your images, it seems like you need the material effect used in your retainer box to be a simple vertical gradient.
Here is an example (although playing with the material itself to achieve the exact gradient needs some experimentation). p.s. You can use a texture instead of a ‘‘gradient node’’ in the material if you prefer.