Anyone knows how to homebrew a 1/3 caster?

So im trying to make a Monk subclass for my player who is 1/3 caster, i assumer the option exists like a fighter Eldritch knight. But when i choose whether the subclass is a caster, it only gives me Half-level and Level, which is not what EK is, i also tried going to Fighter and creating a subclass, but it only gives me those 2 options on a fighter too, i have many supplemental books, but not the core PHB (its shared to me by a friend in campaigns) so i have no idea how they made eldritch knight 1/3 caster, any advice here?

There's a guide to this here: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/119032-how-to-add-spellcasting-to-subclasses-for

Note that while it says at the top of that thread that it's not possible for Barbarian and Monk, later in the thread someone from D&D Beyond staff does say that has been fixed.