Not like other System Center products. Service Manager does not have an option to insert the product key to convert the evaluation mode to licensed mode if the server is running in trial period of 180 days. However, Microsoft has provided a tool for SCSM 2012 for several cases to add the key in to the server. Any way I have tested this in one of my SCSM 2012 R2 Secondary Management server to add the product key after installing the management server. It worked successfully for me and in case if you try it out this is something you have to take your own risk and make sure take the backup in case of an issue. This tool call SCSMChangeProductKey tool. Below screen shows how it inserted the product key in to my Secondary SCSM Management server.
The command which you need to run in Command shell is as below and make sure you run the Command Prompt in Administrator mode.
SCSMChangeProductKey.exe -productkey xxxxx-xxxxx-xxxxx-xxxxx-xxxxx [-dbUser] [-dbPassword]