I installed the Microsoft keyboard language for Japanese on my PC. Once you load the game, switch to JP keyboard.

It has half-width alphanumeric for typing in English, and then you can switch to hiragana, half/full-width katakana, and half/full-width alphanumeric. You can start typing in hiragana, and then press spacebar to pick the correct kanji you want to use.

There's a shortcut way to switch between half-width alphanumeric and hiragana, but I can't remember what it is off the top of my head. Also, there's probably a way to get the default keyboard to JP when you load FFXIV too. Hope that helps!

がんばって！