Footnotes appear at the bottom of the screen and can be added to slides with the ^ Footnote syntax.

Use the Add (+) button to add via the menu.

The text will be selected ready to replace.

On screen, this looks like so:

If you add extra content, the position of the footnote will be adjusted accordingly.

You can add a number to a footnote and it will appear in superscript format, by placing a number after the caret(^), before the space.