Once created, ArcGIS Hub pages are not automatically added to older Open Data sites.

Workaround

1. Select the Page icon, create a new page, and save any customization made.
2. Navigate back to the associated page window.
3. Select Link Page.
4. Input the name of the newly created page > Select > Link Page
5. The page is now found in the Associated Page window, and can be opened from the ArcGIS Hub site.

Steps to Reproduce