Once in the 'Server Manager' Expand Configuration --> Local Users and Groups>Users.
Right-Click 'Users' then select 'New User...'
Make sure to fill the user name and create a strong alpha numeric password. (For the best security practices always include one number and one letter with a mix of capital and lower cases.)
You may also check 'User cannot change password' to ensure the password is not changed.
Check 'Password never expires' to prevent being locked out of the server.
Now that the user has been created you want to make them a part of the 'Remote Desktop Users' group.
Right-Click the new user and select 'properties'.
Select the 'Member Of' tab then click 'add'.
Click 'Find Now'.
Scroll down the list of users and select the 'Remote Desktop Users' group.
You should now see the 'Remote Desktop Users' group in the 'Member Of tab'.