The general style guide is to "write it how you say it":

https://en.wikipedia.org/wiki/Article_(grammar)#Indefinite_a...

https://www.chicagomanualofstyle.org/qanda/data/faq/topics/A...

Of course there will be variants where you'd just have to make a decision (e.g. how do you pronounce SQL?)

One note: technically, 'HN' is an initialism; acronyms are special cases of initialisms that are pronounceable.