Using Ubuntu 18.04: On VestaCP installation, how to get result (https://subdomain.domain.tld:8083) instead of (https://123.312.321.23:8083) [closed]

I’m trying to install VestaCP on Ubuntu 18.04 but keep getting the following result after successful installation:

https://123.231.312.23:8083 instead of:

https://subdomain.domain.tld:8083

What do I have to do get this result: https://subdomain.domain.tld:8083?

I set hostname to server1: $ sudo hostnamectl set-hostname server1

then edit host file: $ sudo nano /etc/hosts

IP_address subdomain.domain.tld subdomain 

After all is done,I get hostname and FQDN as shown below, which is as it should be:

$   hostname server1 $   hostname -f server1.domain.tld 

But then, the result after successful installation: https://123.231.312.23:8083

What am I doing wrong?