You have embedded your form on a web page using the Iframe
publishing option. You are trying to share the link to that web page on Embed Code
option provides. Paste it on your web page. Now try to share the link again. You’ll see that the text referring to iframes does not appear anymore.
If you don’t have much textual content on your page, add the blurb that Facebook/LinkedIn should pick up in the meta description tag of your web page.
If you want Facebook/LinkedIn to also pick up an image, then add one on your page. Make sure to provide its source in an tag, not as the background image for another element.