Translation:Just a moment!
Seems like "Hold on!", "One second!", and "Wait a minute1" are all things a person might say instead of "Just a moment!" that serve much the same function.
Can you say, "Unu momento!" in Esperanto, like you do in English, or is that too literal?
"Unu momenton!" should work - keep the accusative.
I think this is the accusative of measurement, the same one that crops up in "tri kilojn peza" or "ni laboris la tutan tagon".
Or it could be accusative because it implies a larger sentence where momento is the direct object, just like "saluton" and "gratulon".
Yes, I understand "Momenton!" as a short for "Atendu momenton!" or "Donu al mi momenton!".
Probably "Donu al mi momenton." If I understand things right, "atendi" isn't a transitive verb and it would probably be "atendu momente".
atendi is transitive (mi atendas vin "I am waiting for you").
But in atendu momenton, momenton is not the direct object; it's in the accusative of measurement.
Compare e.g. the quote Adiaŭ, sep jarojn atendu min! from Zamenhof in the PIV dictionary entry for atendi -- the direct object is min and sep jarojn (in the accusative) describes for how long the listener shall wait for the speaker.