you don't. the upgrade path of w10 upgrade doesn't involve distribution of w10 key. future fresh installs after successful upgrade is keyless.

yes there are problems with the current way of doing things for w10 upgrade. with the new insider preview (build 10565), you will be able to use keys from old windows versions to activate w10. that means in near future (like november this year, according to this one, http://winsupersite.com/windows-10/details...ion-threshold-2 ), when you do a clean install with the latest build from ms (th2), as long as you have legit old windows keys, you can use that to activate w10.