Hi Hope you are well,

Popular items are personalized for each user and determined by how frequently they order a particular item.

You likely can't manually add items to this list unless you create a separate version that allows you to force specific items to appear for everyone.
https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0952747

To better understand how records are displayed, which widgets are involved and how they are ordered:

https://www.servicenow.com/docs/bundle/yokohama-platform-user-interface/page/build/service-portal/co...

How to increase the amount of popular items to be displayed in the widget options instance:
https://www.servicenow.com/docs/bundle/utah-platform-user-interface/page/build/service-portal/concep...

If my answer helped you, please let us know by marking it as correct and useful.

Thank you
- Carlos Petrucio