This answer, from a similar question two years ago, might help:
"When a paid user (Basic or Plus) embeds their form into their own web page then the powered by link (and other links) at the bottom of the page will automatically disappear. You can get the embed code by going to the Distribute tab and clicking the Embed button. Copy the code and paste it in your HTML page. When you view your HTML page you will not see our links at the bottom and the URL in the browser window will be yours - its completely white label."