There is no need for an unsubscribe link in a welcome email. The email is deemed as a transactional confirmation and permission implied by providing an email address.
The problem with us even attempting to add an unsubscribe link is there is no database to unsubscribe from when we send the email.
If we're provided with an unsubscribe link that to your database so that when you receive the email address, you know not to send anything to it, then we can add that link to the email. However, as this first transactional email is a one off piece of communication, the unsubscribe request is probably better catered for by options on the form that give the supporter an opportunity to opt out of email communications beyond the first confirmation, or through your emails to the donor once you start regular communication with them.