Aha that's just an inline footnote, we support both in Supernotes. So you can quickly write ^[Name of Reference] (that will auto assign it the number 1 once rendered) rather than [^1] ... [1]: Name of Reference.
Footnotes aren't part of the original Markdown specification (https://daringfireball.net/projects/markdown/)
Small note: on that link the footnote syntax is wrong (unless Supernotes uses their own specific syntax) in that it should be [^Footnote] (caret inside the brackets).