I would never paste my API key into an app or website.
If OpenAI wants to support use cases like this, which would be kind of cool during these exploratory days, they should let you generate "single use" keys with features like cost caps, domain locks, expirations, etc
If you're not using the API for serious stuff though it's not a big problem, as they moved to pre-paid billing recently. Mine was sitting on $0, so I just put in a few bucks to use with this site.