In order to verify your application we need the Application Client ID and the possible redirect_uri values you will be using with the authorization flow: https://www.cronofy.com/developers/api/#authorization.
This helps us guard against phishing attacks by controlling where we return authorisation codes to for your application. We don't support wildcards but can support multiple values.
This Stack Exchange article is a good explanation of why we do it. What is the purpose of OAuth 2.0 redirect_uri checking?
Once you're application is in production then this also will start billing. The free tier only applies whilst your apps are in development mode. So if you haven't entered your billing details yet, you'll need to do that as well before you go live. https://app.cronofy.com/billing
When you're ready to go, just contact us via chat or email (email@example.com) and we'll get your app live.