How to embed an external Form into Eloqua email only?

How to embed an external Form into Eloqua email only? There is no business need for data integration with Eloqua due to customer data principles. Take an example, there is a webinar register form. Just use Eloqua email to spead the form and to get more registrition. HTML email for this? any practice for reference? And any side impact on email performance, like delivered rate dropping? Thank you.