How can I create an Extended (custom) Property with EWS?

I have created a little demo application which is able to create a meeting request by using the Exchange WebService (EWS). I would also like to add some custom properties to this item but up to now I have only found examples which use the Exchange Managed API, which I won't use. There must be a way to create these properties by only using EWS...

Does anybody know how and can provide a little co开发者_开发技巧de example?

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