I’m using Xperia 10ii with Jolla-xt9
Type out the following sentence containing this not so commonly used English word (or replace it with an uncommon word in your native language);
Jentacular jentacular jentacular
That’s it!, by the time you are halfway through typing the third word, you should see your new word appear in the predictive list.
EDIT: perhaps to actually finish typing the the third word and go on to typing it a fourth time. I can’t quite nail down the procedure, but it seems to be working at least for English.