The standalone 'markdown' utility is perfectly happy with an external link inside a `
`, e.g.:
# Review of [Dwarf Fortress][]
...
[Dwarf Fortress]: http://www.bay12games.com/dwarves/
produces
but when I try to use this construct in an ikiwiki page, I get
Review of [Dwarf Fortress][]
It works fine with h2 and deeper. The square brackets also appear in the output of an [[ikiwiki/directive/inline]] directive in archive mode, I haven't tried non-archive mode.