necessary data rewind wasn't possible

There is no way to increase the length of a captured packet in the Roku's tcpdump utility.

You would generally send the cookie via the second request via a cookie as theEndless points out.

Here's an example:

        request = createObject("roUrlTransfer")
        port = CreateObject("roMessagePort")
        request.setPort(port)

        timeout% = 1000 * 5

        resArray = CreateObject("roArray", 5, false)
        request.setUrl(loginUrl)
        if (request.AsyncGetToString())
           event = wait(timeout%, request.GetPort())
           if type(event) = "roUrlEvent"
               resArray.push(event.GetString())
               resArray.push(event.GetResponseHeadersArray())
           elseif event = invalid
               print "AsyncPostFromString timeout"
               request.AsyncCancel()
           else
               print "AsyncPostFromString unknown event"; event
           endif
        endif

        cookiesArray = CreateObject("roArray", 2, True)
        for each elem in resArray[1]
            for each obj in elem
                if obj = "Set-Cookie" Then
                    cookiesArray.push(elem[obj])
                end if
            end for
        end for

        request2 = createObject("roUrlTransfer")
        request2.setPort(port)
        request2.seturl(userDataUrl)
  
        for each cookie in cookiesArray
           request2.AddHeader("Cookie", cookie)
        end for

        userData = request2.GetToString()

--Kevin