when you put a text box on the form, it comes with a label.
if you dont have one, copy one on the form, select the text box, paste.
if you dont have any labels, click the label tool, put it on the form.
Not sure why you want to do this but you can use the fact that you can format controls differently depending on whether they contain a value or not
For example, if a number field is empty, you could format it to show 'Please enter a number'. For a company name text field, you could use 'Please enter the company name'