I don't believe it is possible to add a popup to the Order Online page. Your page should automatically display a popup, though, alerting visitors to choose one of your fulfillment options. Choosing pickup will display the available pickup locations that you offer.
Would the Announcement Bar option work for you instead? You can display your notification in a a banner that displays at the top of your website, and change the settings option for the bar when you setup the option. Click on Website > Popup > Build an Announcement to get started.
