Add the following code: 

<iframe src="https://www.matchinggifts.com/xxx_iframe;" height="200" width="400" frameborder="0" scrolling="auto"></iframe>

This URL is given to you by HEP data with your organization's unique URL. 

If you are embedding this is a secure donation form, use https:// when coding the iframe.  When inserting into a non-secure page, use http://

If you have not started the process of setting up matching gifts, review How do I set up matching gifts on my website?