Product messages can be configured from each phase's section within the selling period rule.
Open the target selling period rule
Open the section for the phase you want to configure ("Before selling period," "During selling period," or "After selling period")
Enter your message in the "Product message" input field
Save
Text formatting: Bold, Italic, Underline, Strikethrough, Text color
Insert link: Set the URL and how it opens (same window or new window)
Variables: Insert start and end date/time