How do you fill a UScrollBox?

I have a UScrollBox and a content that is smaller than the UScollBox. I would like to make that content fill the UScollBox completely in the scroll direction. Is there a way to do it?

At the moment a vertical scrollbox can only be filled horizontally, but not vertically. I need to vertically fill a vertical scrollbox.

No - the ScrollBox relies on the desired size of the content to handle the scroll direction size. If you need something more custom you may want to look into writing a new Slate panel that handles scrolling and sizing things to suit your needs.