You can manually add a single computer to the list of computers to be monitored by right-clicking the "SERVERS/DEVICES" item in the navigation pane in the "Servers/Devices" group. Doing so will first show the dialog below to collect the server name.
The "This is a Windows computer" gives the system hints about how it might need to authenticate with the remote computer. "Use WMI..." tells the system whether to attempt to query the new computer with WMI (Windows Management Instrumentation) in order to create a more detailed status report. Unchecking this value, even for a Windows server, will have no other effect other than a slightly less detailed status report. You can change these two settings later by right clicking on the computer and selecting "Set Server Type".
After the server name is entered, the credentials used to monitor the server are requested using the dialog below. You can enter credentials, or just monitor the server using the login that the monitoring service is already using. You can change these credentials later by right clicking the computer in the navigation pane and selecting the "Set Login Credentials" menu item. The help page Remote Monitoring Hints has some advice and information about user accounts when monitoring Windows servers.
Although this is an easy method to add a single computer to be monitored, it is even easier to press the Easy Configuration button and add the server(s) that way because you can: