add parameters to form url before submit

I need to add a path to a file to the form url before submitting. I created therefore a text field and want to add the text to the url without php.

can somebody help?

the existing code:

<form action="http://ift.tt/1A2lXNf" method="post">
      <input type="text" value="" size="30" name="filename">
      <input type="submit" name="submit" value="Submit" class="continue-button">
</form>