Add a link to planner via addNavLink with a SharePoint site script?

Manually, it's easy to add a link to planner in the UI: edit the nav, add a link, and choose "planner" in the dropdown in the "add a link" dialog. Simple. The same option doesn't seem to be available via the addNavLink option with Site Scripts, as the https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/site-design-json-schema#add-a-navigation-link only mentions static URLs. Is there another way to determine the URL for the default Plan for a newly created site, as to be able to add it to the left nav as part of the site creation process?