Hide custom field on edit screen

[rajey]

Hi,
I'm trying to hide a custom field (Single List Picker) based on another custom field.
I had a go at using behaviours and it doesn't seem to work.

The custom field I'm trying to hide is only on the edit screen (not on the view screen).

Here is what I have tried:

def cField_1 = getFieldById("101")
def cField_2 = getFieldById("102")

if(cField_2 .getValue() == "MY VALUE"){
   cField_1 .setHidden(true);

} else {
   cField_1 .setHidden(false);
}

 I have looked at other posts and using behaviours it looked like they were able to hide a custom field.

I also tried this for the field I'm trying to hide: