AD Auto Proxy Address

You can add address policies via exchange management console.
https://technet.microsoft.com/en-us/library/bb125155(v=exchg.141).aspx#UseEMC

There are shorthand policies formats that can be applied

https://technet.microsoft.com/en-us/library/bb232171(v=exchg.160).aspx

Example
%g.%s@contso.com

Note:  once created your can push to all existing accounts.

Hi yo_bee I am on Oultlook 365, I found this that may help: -
https://www.experts-exchange.com/questions/28230622/How-to-add-SMTP-address-to-proxyAddresses.html

what you are showing is with set-ADUser...

That is always an option

But you asked question that you need to add proxy  address when you create new user...isn't it?

Yes, we did this with an exchange policy when we had an on premise set up. I don't think you can do it with 365....

I don't think you can do it with 365....

Yes, you can do it with -otherattributes switch as shown earlier, for that exchange is not required

Thanks for the help.