as a quick reply, it seems that from the first line on, your lines and wrong...

if you put something in the event string "" you need to double the $-sign, otherwise it gets just printed to console window.

in this case, it seems easiest the way shawn said before.

remove the events from the radiobuttons and place one in ok-button.
code:
$ok_button.onclick="ok_clicked()"
then place a function at the bottom of the script file:
code:
function ok_clicked()
select
case $optionbutton1.value
 ;do something
case $optionbutton2.value
 ;do something
case $optionbutton3.value
 ;do something
case 1
  $=messagebox("You must select a value!","ERROR",20) exit 1 ;exits the function
endselect
$form.hide
endfunction
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