You cannot do this via the ShapeSheet directly. You must use automation
code or Insert / Field to do this first, and this will result in the
ShapeSheet section being added.
It is normal practice to have a User-defined cell, called say
visCustomLabel, and then to insert this a a fieldinto the text block.
You can then modify the formula in User.visCustomLabel to suit.
This is how the Label Shapes add-on under Space Plan works.