Just found a cool way to insert a formula into a text box in just a few simple steps.

You need to select you text box, type = into the Formula bar, and pick the cell or formula you want to link to your text box.

I learned it from this guide on how to create, edit, and remove a text box in Excel: https://spreadsheetsuperstar.com/basics/how-to-add-edit-and-remove-a-text-box-in-excel/

Hope this helps someone!