On a form right?
Field type "Display" ?
OnClick tab?
at the bottom is an option
"onClick: [JavaScript]"
This option is disabled if the URL or Procedure options are set.
ie when clicked you can either
a) call a hand-coded URL or
b) Call a Procedure (like a browse or form) or
c) Execute some JavaScript.
Actually, since (a) and (b) are really just pre-determined JavaScript calls, the last option is a superset of the other two, however you would need to make the necessary call yourself at the end of your code.
Cheers
Bruce