Dynamic variable in domain name

I tried a dynamic variable in URL as shown in the example in the documentation ddosify -t target_site.com/{{_randomInt}} and it works as expected.

But if I try the dynamic varilable in the domain like following ddosify -t https://{{_randomDomainWord}}.target_site.com i get following error: err: target is not valid: https://{{_randomDomainWord}}.target_site.com