@misc{ wiki:xxx, author = "Wikimedia Commons", title = "File:Government Jack of the United Kingdom.svg --- Wikimedia Commons{,} the free media repository", year = "2024", url = "https://commons.wikimedia.org/w/index.php?title=File:Government_Jack_of_the_United_Kingdom.svg&oldid=839364122", note = "[Online; accessed 28-June-2024]" }
\usepackage{url}
somewhere in the preamble) which tends to give much more nicely formatted web addresses, the following may be preferred:
@misc{ wiki:xxx, author = "Wikimedia Commons", title = "File:Government Jack of the United Kingdom.svg --- Wikimedia Commons{,} the free media repository", year = "2024", url = "\url{https://commons.wikimedia.org/w/index.php?title=File:Government_Jack_of_the_United_Kingdom.svg&oldid=839364122}", note = "[Online; accessed 28-June-2024]" }