From the menu select Registries then click Add registry. Ensure Custom registry is selected.
In the Custom registry details section, enter the registry name and URL (both mandatory). If authentication is required, toggle Authentication on and enter the username and password.
When you're finished, click Add registry.
Simply adding a registry will not make that registry available to non-administrator users. Once you have added the registry to Portainer, you will need to add access to the registry within Host / Swarm / Cluster for each environment that needs access.