Extracting domains from a CSV URL column using Miller

Having CSV content similar to this:

Family,URL,IP,FirstSeen
Pony,http://officeman.tk/images/admin.php,207.180.230.128,01-06-2019 
Pony,http://learn.cloudience.com/ojekwaeng/yugo/admin.php,192.145.234.108,01-06-2019 
Pony,http://vman23.com/ba24/admin.php,95.213.204.53,01-06-2019 

I'm aware that the URL column can be selected using:

mlr --mmap --csv --skip-comments --headerless-csv-output cut -f 'URL'

How could domains be extracted using Miller w/out piping to other commands? **Desired Output:**

officeman.tk
learn.cloudience.com
vman23.com