To add "sample text" in a text field, right-click on the text field and select Patterns > Display.
Check the "Allow Empty" box and enter your sample text. Granted, it will be in the same font/default color as the text being entered; however, you can add a couple of Action Builder routines to change font colors, such as making the sample text a light gray and entered text as black.
To do that, three Action Builder routines are necessary: 1) Initialize the text field and set its font color to gray; 2) when text field is entered (i.e., cursor is placed in it), change the text field color to black; and 3) if the text field is empty, set the font color back to gray.
Using "Allow Empty" is different than adding a default value--default sets the actual value of the text field, whereas, displaying text when the field is empty does not alter the field's value whatsoever.
Action Builder will automatically write the JavaScript necessary to do those actions. To see the script (and learn how to write script on your own), click on the text field and open the script editor window. [If the script editor is part of the tool bar, just double-click on it and it will pop-out so you can expand the window.] Click the drop-down arrow beside "Show:" and select "Events with Scripts". Font colors "0,0,0" will be black and font colors "192,192,192" will be gray. If you know a little JavaScript, you can also change the font itself by editing those Action Builder sequences.
BTW, this works for other types of entries too: numeric fields and custom fields (emails, phone numbers, SSNs). Just set the sample display for the empty field to a sample of whatever you're expecting the user to enter.