Retrieve preview pages URL to use as environment variable in the dashboard

Is there a way to use a placeholder/variable in the Environment variables dashboard to account for the preview Pages URL?

Something like:
SITE_URL = $CF_PAGES_URL (but via dashboard)

Why not access the CF_PAGES_URL environment variable inside the build rather than SITE_URL?

Alternatively you could set SITE_URL=$CF_PAGES_URL in the build.