Hi,
For the normal API there is no "machine to machine" authorization flow, there is only the code grant flow, to get the code you need a user to logon.
For the neXtgen, you do not need to set the application to public, you can use "invite only".
Please let us know what you want to do and what you are having issues with.
How to connect is described in both those documents for their respective endpoints.
We're guessing that the neXtgen is what you have been looking at the most, in that case, please only read the documentation for neXtgen for now to not confuse the two different authentication methods.
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