I don't know the answer but I think that you have a very beutiful name.
Even Harry Potter can't do it... (nt)
You can't put a formula in a textbox, but you can put the result of a formula in a textbox. Enter the formula in a cell and then name that cell(It can be hidden or on another sheet). Now create your textbox. While in the edit mode, click in the formula bar and enter =cellname. Click outside the textbox. The result of the formula will now appear in the textbox and will update if the results change. You can be creative in the formula bar and concatenate text with the reference cell to display a sentence.
Hope this helps