Check box to copy text from 1 field to another

I am trying to use a check box to copy the tech from a billing address text box to a shipping address text box only if the check box is checked.

I am new to using java and building forms in acrobat so please understand.

I have googled and tried to implement the following script in the shipping address Text field properties under format. but it doesn't work

if (this.getfield('checkbox').value =='Yes'){SHIP_TO.value = Bill_To.value} else {Ship_To.value =""}

thank you for any assistance