Thank you for reaching out to the Community. That is a great question! You can add custom fields to your Sign-Up Forms by following these steps. Once you have created your custom field, you will then be able to add it to your Sign-Up Form in the Contact Fields section.
Unfortunately our system does not currently have a feature where you can link to an outside source on the Sign-Up Form. I will certainly submit your feedback to our developers. It's great insight for our developers to review as they map out future updates for the product. I have tracked it in your account for them to see. I also encourage you to check out our Feedback area of the Community where you can post any additional feedback you have and chime in with other customers sharing their ideas.
Please let us know if you have any other questions. Thank you and enjoy your weekend!