A few points to make first:
1) iframes are depricated. Not recommended.
2) Some people would frown upon "linking in" a page from another site that is not yours, kind of like hotlinking images only doing so with the whole page.
3) Depending on what kind of form that is, what kind of information is collected and how it is submitted, you could get listed as a fraud/scammer unless you make it clear where form is from. I would say just don't do it.
4) iframes will show the whole page you link to. Like the name implies, it's just a frame, you can set the size and position but otherwise the page will be served exactly as if you were to go to it normally in a browser.
5) If you know the construction of a site - say the form is located in an include file somewhere that you could use - you would have access to the code and should be able to copy it over. No need for iframes!
6) I would suggest making it clear that anything you "link in" with a frame is clearly marked as part of another site, if you do so. But I still don't recommend iframes AT ALL.
But even after all that WHY wouldn't you create your own form? There is no reason you should be pulling in someone else's form in a frame. Better to just link to the original page, take a screencap for a placeholder if you must, or better yet, make your own form. And don't be afraid to ask for help here if you need it.