[Zope] dtml modification of request

I have a DTML document containing a form with input feilds named input_a and input_b. I want to pass a calculated value along with these two items into the target of the form. My understanding was I could wrap a <dtml-let input_c="foobar"> around the form and get all three values visable in my form destination object. How can I pass this variable into the request?