Help text for Multiple lines of text

Give the Multile Lines of Text control a JavaScript Client ID Variable.

Then in the Custom JavaScript under Settings, enter this code:

NWF$('#' + Multiline).attr('placeholder',"Some text that you should see before you enter the field.");

Obviously, you can change the text to say what you want, but make sure to use the JavaScript Client ID Variable name in place of mine, which I called 'Multiline'