Usage: 2 forms in 2 verses
Form Translated As     In Verse(s)
εκουσιασμώ voluntary offering LXX.1Chr.29.9
εκουσιασμού a voluntary offering LXX.Ezra.7.16

Liddell-Scott-Jones

ἑκουσιασμός (hekousiasmos) a voluntary offering G:N-M

ἑκουσιασμός-ασμός, ,

free-will offering,[LXX+2nd c.AD+]

LSJ Scripture Refs: LXX.Judg.5.2