If you have login details for Disney+ then I would assume that you would log in using those details when in Roku. If, however, you cannot get those details and/or Disney+ is intrinsically linked to an Apple TV box/account then maybe it is impossible.
How did you sign up for your Disney+ subscription? Did you purchase the subscription through Roku Pay, or did you sign up with the channel provider directly?
If you need further assistance with account and billing-related issues, please reach out to our support team directly here for your 'Account or billing' issue, here: https://support.roku.com/contactus. They will be able to further assist you.
If you need more information about the Disney+ bundle, you will need to contact Disney+ support directly. You can reach them here: https://help.disneyplus.com/csp