Oknoda is free, and it stays free. If you would like to chip in anyway, this is where — it goes towards the server the world runs on.
What it does not buy
Anything in the game. There is no shop, no supporter tier and no perk — a donation changes nothing about your account, and nobody in the world can tell who has given. That is on purpose: the FAQ says why the game costs nothing, and this page does not change the answer.
Other ways to help
Play it, and bring a friend. Report what breaks, in the game's chat or on Discord. A bug found is worth more to a one-person project than most things money buys.