You can do that via the Control Panel applet: Security.
Add a new user there eg. “User” and assign the TS Session to this user account. Now put this user in the Guest group.
Don’t forget to assign a good password for the administrator account.
After that define the User to be autologged in.
Leave the Security applet and reboot.
The user should be autologged in with guest credentials and the TS session should start directly.
To interrupt the autologin press ALT+F4 and log in then as the administrator.