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You really shouldn't use API keys with ProGet, actually. From the docs:
An API Key is used by NuGet client tools (such as nuget.exe and NuGet Package Explorer) as an authentication mechanism for publishing packages. It makes a lot of sense for the NuGet Gallery, as it's a community-run site where anyone can publish packages they own.
In enterprise environments, the notion of individual package ownership doesn't quite translate, nor does the usage of API Keys. With ProGet, a principal (user or group) either has privileges to publish a package to a feed, or does not. This allows ProGet administrators to more easily control access and usage through LDAP and Groups instead of through community mechanisms like API Keys.
So, if they can authenticate to the feed (using basic auth or windows integrated auth... or even an api key in the form of user:pass), and they are authorized, then the can push a package.