Authentication depends entirely on the type of authentication/oauth client you've created. If you're intending to login as an agent, you're likely looking at either the implicit grant or code authorization flows, which are documented here:
https://developer.mypurecloud.com/api/rest/authorization/. Generally they involve displaying a login screen where the user provides their credentials; this login screen then redirects back to you application and return the authorization token (and refresh token in the case of code authorization).
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Richard Schott
Genesys - Employees
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