You will need to install the Smart Lock Guest Access app. Once installed, you would open it and in the upper left hand corner of the screen you will see Locks with a black pull down button that you can click on and select Lock Codes.
Side note: availability of Smart Lock Guest Access depends on what region you are located. If in US, it is available. Other regions outside of US, it may not be available to install.
If not available, an alternative is to use LUM from @RBoy apps or you can use it in place of SLGA if you desire. It does offer more functionality.