Update hidden field

I am trying to update a hidden field in Design studio.  I am new to Design Studio so please forgive. 

The hidden field needs to be updated when the select12 is "S".  How would I do this?

function FORM_OnBeforeTransaction()
{
     var vList = lawForm.getFormValue("select12");

    if (vList == "S");
//         UPDATE ?hidden56? = ?y?;
           return true

}