This is a paid feature of Ship. Once you upgrade, you will find a toggle option in the settings that allows you to submit it as upcoming.
If you plan to segment your Ship subscribers then upgrading is also required for this.
This was confusing to me as well! You can use the free version of Ship to create an upcoming page, and share that link out. So you do not need to be subscribed in order to create the page and share the link, you just need a subscription in order for it to get listed in the "Upcoming" section of PH.
FlexPDF