function set_field_value(){
    echo "<script type='text/javascript'>
        document.getElementById('acf-field_5cd68b4d12a20').value = 'test123456';
    </script>";
}
add_action('admin_footer', 'set_field_value');

function feed_custom_field($html, $id) {
    $src  = wp_get_attachment_image_src( $id, $size, false );
    $image_url = $src[0];
    echo "<script type='text/javascript'>
        document.getElementById('acf-field_5cd68b4d12a20').value = $image_url;
    </script>";
    return $src[0].",";
}
add_filter( 'image_send_to_editor', 'feed_custom_field', 10, 9 );