Email clients consider having signup forms within emails a security risk. While some email clients warn you of potential danger, others outright disable the signup forms. Therefore there is no option to add a signup form in your Benchmark emails.
Instead, we recommend sending your contacts to a signup form on your website. This is the safest, most reliable solution to pairing an email message with a form. In addition, more people will see it and be able to use it. As a result, engagement will increase.