Symphony.

In my case, the link in the backend goes to http://localhost/blog/post instead of http://localhost/sitename/blog/post.

This can be fixed by concatenating the URL constant to the anchor href value.

$anchor = Widget::Anchor(
    $this->get('anchor_label'),
    URL . $data['value']
);

If this change is made at line 115 and line 168 of field.entry_url.php, the link will work for an install in a subdirectory, assuming that the path from the root URL is saved as the field value.

/path/to/page/{entry/title/@handle}/