In most cases, the server name is going to the computer name where the SQL Sever is located followed by \REALNEX. You can click on the three dots to right of the drop down menu to see if the server name appears in that menu. Alternatively, on the server machine right-click on This PC in Windows Explorer and select Properties. The PC Properties window will show the Computer Name here. For example, if your Computer Name is User-PC, your server name would be User-PC\REALNEX
If you are unsure what the instance name is, you can find it in Computer Management. To open Computer Management, click the Windows Start Menu or the Windows Key on your keyboard and type Computer Management, and click on the tile that appears. Once open, follow this path: Computer Management--> Click Services and Applications--> Click SQL Server Config Manager--> Click SQL Server Services. The instance name will be shown in parentheses.