I've not found a way to get the SiteId of the PageUrlField if using the URL selector as the required parameters are not returned, only the Url, i.e. ~/name-of-page. All the Kentico methods appear to require the NodeAlias and/or NodeGUID and/or SiteId and/or another combination of values.
For my specific use case though, I've been able to get the NodeLinkedNodeSiteID from the CurrentDocument. This makes an assumption that the value of the PageUrlField is within the same site as the non-linked page.
{% GetAbsoluteUrlBySiteId(PageUrlField, CurrentDocument.NodeLinkedNodeSiteID) |(identity)GlobalAdministrator%}
Still looking for a solution to get the Absolute Url from both the URL Selector or Page Selector control types when the value is to a linked-page.