You need to add a CNAME record wherever your site is hosted.
If your site is in Hostgator, follow these steps:
Step 1: Log in to your account in https://www.hostgator.com/
Step 2: Visit ADVANCED DNS ZONE EDITOR.
Step 3: If you want to use login.yourwebsite.com as Subdomain in your K2 White Label setup, create a CNAME record and enter login in the Name (Host)field.
Step 4: Enter dns.kallzu.com in the Value (Points to) field.
If your site is in cPanel, follow these steps:
Step 1: Log in to your account.
Step 2: Click DNS Zone Editor.
Step 3: Click +CNAME Record near the chosen domain.
Step 4: If you want to use login.yourwebsite.com as Subdomain in your K2 White Label setup, enter login in the Name field and put dns.kallzu.com in the CNAME field.
Once done, follow the rest of the instructions we provided in the Help video here — https://kallzu2.kallzu.com/white-label/ to complete the setup. Don't forget to click Update to save the changes.