Placing caption name into another field as text

I have a group of checkboxes, and to the right of each is a blank text field eg:

 BRC ________________
 EMS ________________
 QMS ________________
etc...

When any given checkbox is selected, I want the caption text to be inserted into the blank text field. So:

X BRC BRC
 EMS ________________
X QMS EMS

I've tried several things, but none work, eg:

----- form1.#subform[1].TechCBCF.CB[0]::exit: - (JavaScript, client) -------------------------------
TechResultsSF.Result[0].rawValue = this.caption.value;

or...
TechResultsSF.Result[0].rawValue = this.caption.name.value;

and a whole heap of other things, including trying different events.

Can someone please put me on the right track?