To create a custom unsubscribe link, either create an "Unsubscribe reasons" page in the Pages module, or create your own external unsubscribe page.
In your e-mail, insert the link to the page as a web address:
Make sure to add the correct parameters for Maileon to connect the unsubscription to the contact. These parameters are contactid=[CONTACT|ID] and checksum=[CONTACT|CHECKSUM]. Note the single brackets for the placeholders, because of the placholders are used in a trackable URL.
The completed unsubscribe link would eventually look like this: