When you say scrollview, do you mean a Unity UI scroll view? If so... check out the "UITest" sample scene to see some masking examples. (It's a bit messy, I know...)
If it's Unity UI, I believe just the parent object needs a Mask attached to it. Then, on the Super Text Mesh object, you can change the Mask Mode from "Inside" to "Outside". The order of objects in the hierarchy plays into masking!
If it's not Unity UI... I'll look into this a bit more! So just lemme know!