1. Anchor a UIWidget (ALT+SHIFT+W) to the bottom.
2. Add a child label to it. Anchor its left to the panel's left, and right to the panel's right.
3. Set the label's overflow to "Resize Height".
4. Add a sprite to be the label's background (you can make it a sibling of the UIWidget, or a child of the label, your choice).
5. Give the sprite a lower depth value so it's drawn behind the label.
6. Set the sprite to be anchored to the label fully (left to left, right to right, etc), with some padding so that it envelops the label and resizes with it.
7. If you want the label to grow upwards as its lines increase instead of growing both up and down, change its pivot to Bottom.