These fields are text only by design. A link can be entered as text and will display as text only (not a clickable link).
As an additional related option, there is the ability to add the MOTD to email notification via the ":MOTD:" tag.
From the TPAM 2.5.921 System Administration Guide:
Message of the day is a brief text message that will appear on the home page of the /tpam, /admin, and /config
interfaces. The message of the day can also be added as an optional message body tag in the email notifications
sent by TPAM. The tag for message of the day is :MOTD:.