Adobe Acrobat - Hiding Text Field In PDF With Java Script

I have a PDF that I'm making fill-able in Acrobat. There's a coment section, where I'm doing a multi-line text field. The PDF already has horizontal lines on it for filling it out by hand.

What I want to do is have a text field over this with a white background, so that when you fill it in on a PC the horizontal lines don't show up.But I still want people to be able to fill this out by hand after they print it if they want. So if this field is left empty I want to set it to be invisible/hidden, so that when you print it the lines would be there for writing stuff in.

I've found various forums saying this is possible with the following java script:

if(this.rawValue == null)

{ 

this.presence = "hidden";

} 

But it doesn't seem to be working.

Can anyone who knows how to do this walk me through it step by step? Maybe I'm doing it wrong. I have Acrobat X Pro and Livecycle Designer ES2, but don't know if I need to do this in designer.

Thank you.