How to pass some custom cookie parameters in fastcgi

I have written a server in fcgi and C and I need to add some custome parameter after I printed some String to request.out. to be clear this is my sample code:

while (1) 
{
    rc = FCGX_Accept_r(&request);
    if (rc < 0)
        break;
    FCGX_FPrintF(request.out,
        "Content-type: text/html\r\n"
        "\r\n");
    //the html page content
    FCGX_FPrintF(request.out,
        "<form method=\"post\" action=\"\">"
        "<input type=\"text\" name=\"num\">"
        "<input type=\"submit\" value=\"click\" name=\"submit\">"
        "</form>"       
        );
    .
    .
    .

    //and somewhere like here I need to add a cookie parameter
    FCGX_FPrintF(request.out,
    "set-cookie:myParam=myValue\r\n"
    "\r\n");
    .
    .
    .
    .
    FCGX_Finish_r(&request);
}

but this ends up printing right to the page.

how can I put it to the start of the buffer?

Thanks for your help.