We should mention the limitations of the 5 UI link field and how to work around the issue.

In that case one of the apps is a Magnolia app then you should look for a compatibility module to do linking.

In the case both apps are custom then create a UI5 version of the UI6 app specifically for the purpose of linking. You can create one line app descriptor via https://docs.magnolia-cms.com/product-docs/6.2/Upgrading-to-Magnolia.html#_migrating_content_type_based_apps as workaround.

`!content-type-m5:myContentType`.