Dynamically update a form

I am trying to dynamically update a form so that when you enter text into one text box other things on the form, like other text boxes change.  This is such a common thing to want to do it seems like it ought to be easy but for some reason I haven't found the solution  I tried things like 

set(car(TheForm->fieldList) "Test")

Here TheForm was a variable set to the form ID.  I got the field list because in my form I have a variable number of fields and don't necessarily know the ID of each field.  In this example I was just trying to set the first text box field on the form to "Test".  That didn't work - the CIW echoed "Test" but the form didn't update.

Can anyone help show me how to update a form dynamically so that when one text field is changed by the user, another text field has its value changed?