Adding a new form field to a page, or a new page including a form, is actually a fairly complex undertaking. Do you have PHP development experience? If so, you should first search the forum for things like "add new page" or "add new field", and also consult the documentation. The Developer's Wiki
may be of help, as would be the OJS Technical Reference
(which, even though it was written specifically for OJS, and is somewhat out of date, has a large amount of information on how and why we have structured our applications).
You may also want to just appropriate the Special Requests form field that is already included as part of the registration page. If you want to add further text as instructions on that page, you can modify the "schedConf.registration.specialRequests.description" message key in locale/en_US/locale.xml.