Click on Create Button.
You will see two (2) sets of code: html code Website and a link for Email. Believe it or not, you do not want the the code for websites. WordPress.com does not let users post web forms so the website code would be stripped. You must use the code found in Link for Emails. This is the link that you want to attach to your actual button image. Keep this window open, as you will need to copy that URL in a bit.