First of all, brohoof to my fellow Dashite!
Now, could you possibly post the code for the textbox? I may be able to work out a solution if I see the code.
EDIT: Reread your post. Looking for such a function now.
Second EDIT: No, there does not appear to be, but I do have a solution. Render the text to a render texture which is the size of the textbox. It'll automatically clip the text that goes out of bounds. Then you can render that render texture where the textbox belongs.