I want to put some subdomain.mydomain.com in another server

i understand in current server, in WHM > Edit DNS Zone > mydomain.com > i just need to add A record like subdomain A 222.222.222.222(another server IP)

before that, i need to set the another server first, so i want to order a reseller hosting for the another server. My question is what domain i need to put in the order form? Is it ok still put mydomain.com?

Because i tried put subdomain.mydomain.com but its says not valid domain. Its standart hosting order form, using WHMCS

and what nameserver i need to use in the another server WHM later > Basic cPanel & WHM Setup > Nameservers, is it still ns1.mydomain.com and ns2.mydomain.com like current server use? Where i need to create the nameserver for the another server?

please help, this should be easy if you have done it before