Using the Recommend feature, you can recommend add-ons during online booking. To set up the recommendation feature, go to Manage --> Business Settings --> Add-ons.
Select the add-on you want to recommend.
In the right sidebar, under Packages - Applies to: click Edit.
Select the packages this add-on can be booked with.
At the right of each package name you can select the star icon to recommend the add-on.
When a client books a package with a recommended add-on, they will see that add-on in the Recommended section above all the other add-ons available to that package: