Check whether that dictionary accepts the s-less version only as an adjective. ;-)
The ticket cost ten Euros.
but
It is a ten Euro ticket.
Note: the dictionary might not mention adjectives but just give an acceptable example (which happens to be an adjective).
(Note: in informal speech the S can be dropped in some cases: "Ten quid? It's not worth a fiver." But I've never heard 'Euro' used like this; maybe I would have - if we used it more in the UK.)