How do I add the Best Possible Time comparison? It does not show up on the preselected list of information tabs in layout settings.

Assuming you ask about the "Best Possible Time" info that is often shown at the bottom, after the splits and the timer; Go to Layout --> Information --> "Run Prediction", and add that.

You will see a new layout section called "Current Pace". To change it into "Best Possible Time" information, go to the layout settings for this new section, and change the comparison to "Best Segments".