If you want to add a text box to your PDF file, then have a look here:
http://stackoverflow.com/a/13191097/[
^]
[EDIT]
I found the reason why it is not working: your HTML code is not well-formed, because the
value
attribute is not closed: there's an apostrophe missing.
Try this:
string sttt = "<input type='text' name='firstname' value='Hello'/>";