1. You go to your ask page and view the source. (Depending on what browser you use.)

How to find the source on Google Chrome: 

>Click the wrench
>Tools
>View source 
2. Look through the HTML code and find this code:

<p><iframe frameborder="0" scrolling="no" width="100%" height="149" src=" you tumblr url "  ></iframe><!--[if IE]><script type="text/javascript">document.getElementById('ask_form').allowTransparency=true;</script><![endif]--></p>

3. Open your customize page and go to your FAQ. You go to HTML and you paste it in.