"Good luck! – Thank you!"
Translation:– Onnea! – Kiitos!
I should add that, at least for me, there are better sounding alternatives for wishing someone good luck than "onnea!". Those are "onnea matkaan!", "lykkyä tykö!", and "tsemppiä!". The latter two are among the countless examples of colloquialisms derived from Swedish, because "lykky" is from the Swedish word "lycka", which means "luck" or "happiness" (just like its Finnish equivalent) and "tsemppi" means "fighting spirit" and is from the Swedish word "kämpa", which means "to fight".
"Paljon onnea" is more like wishing someone a lot of happiness (though another meaning of 'onni' is luck; it depends on the context) and that's why it's used on birthdays and such. It doesn't really work for wishing luck - in that case just 'onnea' is better (or e.g. "lykkyä tykö" [luck to(ward) you]).
Even with just "Onnea!" I think only of "happiness" when I say or write it on a happy occasion, such as a wedding or a birthday. They have already been lucky to get to that situation, now I wish them to be happy, although luck might also be needed for that. Before the wedding ceremony, I may say "onnea!" in the sense of "good luck!"