You can simply use the API Key and Access Token that is generated through the MAshery I/O Docs. The OAuth flow is provided specifically for use with applications that want to connect with multiple Constant Contact accounts. If you are working to create console applications that would be expected to access multiple unique Constant Contact applications, you would likely need a database and a secondary application to handle the OAuth flow so that you can gather the Access Tokens for those accounts.
Also as a last note, I want to clarify that an API key is used to identify your application, and an Access Token identifies and authenticates a Constant Contact account. There should be 1 API key per unique application, and there can be any number of Access Tokens that involve a given API key.
If you have any questions about this, please let me know!
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