How to host main domain (like whmcs) on another server without changing Cwp name

My domain example.com
My cwp name server ns1.example.com ns2.example.com
How to host main domain (like whmcs) on another server without changing Cwp name server

  Please help me

As I understand you want to make access to your billing (WHMCS) through the URL like:

whmcs.example.com

Right ?

If so then create the virtual host/account with the domain name/subdomain whmcs.example.com on your other server then edit the DNS zone of example.com on your primary server and add the DNS record like:

Name: whmcs.example.com.
Type: A
Destination: xxx.xxx.xxx.xxx

where xxx.xxx.xxx.xxx is the IP address used by whmcs.example.com on your second server.