How to Hide a group box in a Form

Hi there

I would like to hide a group box in a form with a name= Cost. This group box has a

name = GroupBoxDD

property name=GroupBoxDD

I tried to get the group box item by using 

var Item = document.thisItem;

var GroupBoxDD1 = getElementByID("GroupBoxDD");

var var GroupBoxDD2 =getElementbByName("GroupBoxDD");

or

var GroupBoxDD1 = getElementByID("GroupBoxDD");

var var GroupBoxDD2 =document.getElementbByName("GroupBoxDD");

or

var GroupBoxDD3 = Item.getElementByID("GroupBoxDD");

var var GroupBoxDD4 = ITem.getElementbByName("GroupBoxDD");

None of the lines give me back the group box item, they all failed.

Also I am not sure where to find element ID, how to use getElementByID function. Seems to me that when I create a form with an element, there is only a name, a label and a property associate with it.

I am looking forward to your kind response

Thanks a lot