inko

There is no -ink-. Esperanto's suffixes are a closed and well-published list, and -ink- is not on it. The resemblance to -in-, the feminine suffix, is pure accident: -in- gives patrino and reĝino and would never appear with a k after it. Ink- is a single root and the word is ink-o, root plus noun ending, nothing more.

The container is the real derivation. Inkujo is an inkwell or an ink cartridge, using -uj-, the container suffix, which also gives monujo (a wallet), salujo (a salt cellar) and cindrujo (an ashtray). Inka is the adjective, inkigi to ink something, inkmakulo an ink stain, and seninka describes the pen that has run dry.

An emperor's word, oddly. The Romans wrote with atramentum; encaustum was the purple-red ink that only the emperor was allowed to sign in, and it got its name from enkaiein, to burn in, because of how the pigment was fixed. That imperial word is the one that survived into the Romance languages and from there into English and Esperanto, while the ordinary Roman word for ink vanished. So every biro in Europe is named after a Byzantine signature.

Writing with it. Esperanto uses per for the instrument, so skribi per inko and skribi per [plumo](/esperanto/words/plumo) are both right, and en inko is an Anglicism worth avoiding. Plumo is both the feather and the pen, krajono the pencil, globkrajono the ballpoint, and inkrubando the typewriter ribbon that a certain kind of Esperanto novel is still set among.