We don't have a way to add a second domain into the same account for testing purposes. Until we build that, if you'd like, you can sign up for a new domain (for example, you could create {yourdomain+dev).
After creating this account your API key for staging will be immediately availalbe under the 'developers' tab in your dashboard.
Once the account is set up, contact support through help@daily.co to make sure you have access to all the same features as your main account, free of an extra charges.