You can’t do this with ACF and it would be difficult using WP to convert a link url to a post id. You might be able to add a filter to “the_content” that looks for urls and tries to convert them to an ID. You might be able to build your own shortcode to put into the content and let them select pages post object ACF field and then get the ID from that, but that’s not something that ACF can do and you’d need to build the shortcode PHP yourself.
