It can be done in several ways:

a) If the work is already registered previously, in the registration management under "Registration" in the "Responsible declarations" section by hitting save on each button as it is completed.
b) If a work is being registered: in the registration process in step 2 of the work data.
c) You can make a responsible declaration of any work from the link at the bottom of the page: https://www.safecreative.org/register-responsible-declaration where the necessary data will be requested and the file containing the work will be uploaded. In this case you are not declaring authorship or other rights, but you can do it later.