Open the target selling period rule
Open the "Before selling period" or "After selling period" section
Enter your message in the "Alert message" input field
Save
When you create a new rule, the following default text is pre-filled in the input field (and shown as a placeholder). If you clear it and save, no alert message is displayed:
Before selling period: "Selling period has not started yet."
After selling period: "Selling period has ended."
Note: Alert messages support plain text only. Text formatting such as bold and links, and variable insertion are not supported.