I have never heard anyone just say, "a moment" in English. We always preface it with "just a-" or "give me a-".
"Hetki" is acceptable on its own in Finnish, but the English translation needs extra words to demonstrate this as standard usage.
This translation confuses me... are they asking for a moment of someone's time because they are busy? Or are they asking you to wait a moment because they are busy with something else? I don't know what a moment is supposed to mean here