I wrote three levels of dialogue for a game, which was a lot of fun, but just happened because I was in essentially a low-level producer role, the lead game designer left, and the studio head was unhappy with the dialogue and wanted it rewritten three hours before submitting the build to the publisher.
So that sounds like just blind luck, but aside from collaborating on indie projects here, if you want to get into professional game writing, similar to game design, it helps to start out in a game studio any way you can, e.g. many start out in QA then work their way into game design or producer positions. And game writing has its own requirements, constraints and conventions (the person responsible recording the voices wanted me to list the dialogue in a spreadsheet in a certain way), so you can learn a lot from just seeing a real project develop.
I also recommend this book from a friend of mine (he was a producer at a publisher of a game I worked on)
https://www.goodreads.com/book/show/26207943-slay-the-dragon