See also the guide: https://www.smarthost.pl/poradniki/hosting/przenoszenie-domeny-do-smarthost-pl/
To connect a domain registered with another registrar:
1. In cPanel on Smarthost.eu server, in the additional domains, add the domain you want to be connected.
2. You should move the website and create the necessary e-mail accounts, ftp accounts or mysql databases.
3. In the panel of the current domain registrar, click "Change DNS" for the selected domain and anter the following addresses:
1. DNS 1 server -> dns.smarthost.eu
2. DNS 2 server -> dns2.smarthost.eu
3. DNS 3 server -> dns3.smarthost.eu
4. Then, you should approve the "Change DNS" option.
After this operation, the domain should change its existing servers up to 24H with new ones.
You can also transfer the domain to Smarthost.eu and pay it with the hosting package, it is necessary in the current registrar to get the AUTHINFO code which entitles you to transfer the domain. Then log in to your account www.smarthost.eu/panel/ and go to the home page www.smarthost.eu. Then select from the menu the option: Domain registration -> Domain transfer and complete the application by providing the previously downloaded AUTHINFO code obtained in the panel of the current registrar.