There are two distinct ways in which you can consume the AccountRight/ new Essentials API and SDK.
Firstly there is the local desktop service which must be installed and running as part of AccountRight and secondly the online cloud services which use standard OAuth 2.0 authentication.
For examples on Authentication, check out our GitHub repo of the SDK here
For some additional reading on what OAuth is please refer to here
If you are unsure of what a redirect uri is, our support team has put together a little video tutorial unpacking this here