If you press the footnote button, you'll see the footnote inspector. Here's a brief explanation of what it does:

The identifier is the small short number linking the footnote. The Reference is the actual text content of your footnote.

The footnote content also supports markdown formatting blocks, such as bold or italic..

Footnote Identifiers

You can use any identifier you want. But if you keep using numerical identifiers, then Hyperdeck understands your footnotes and makes sure they're always sorted correctly.

This means that if you have footnote identifiers 1 and 2 on a slide, and you insert a new one, Hyperdeck will automatically assign it identifier 3.