Navigate to Virtual Servers and click the server you would like to add an IP to
Navigate to Networking > IP Addresses
Click Allocate New IP Address
Select the network interface the IP address should be assigned to. Then click Add IP Address
In this step you have the option to select a specific IP address. If not selected, the next available IP will be assigned.
To automatically add the IP address to the server click Rebuild Network
Not all templates support rebuilding the network configuration live and may require a reboot.