Referencing parent entity
I came across this post when facing the same situation so I am posting the solution I found in case someone else needs it.

The only way I found to solve this was declaring a custom token using hooks. This post was really helpful: https://drupal.stackexchange.com/a/78498

Once both hooks are implemented, you will only need to manually load the parent to get the data you need, whether it is the parent URL or the parent title. After that, just use the token you just created and voilá!

Hope it helps!