SPOnlineRESTClientCore (nuget): .NET Core 3.1 SharePoint online REST client with authentication using only username and password, without any Client ID or Client secret.  Offers only three methods: Authenticate - providing username, password and endpoint, which you want to call your REST request upon ExecuteGetRESTCall - execute supplied REST GET call (part of url starting /_api/) ExecutePostRESTCall - execute supplied REST GET call (part of url starting /_api/) with Json body  Works well with Azure Http triggered function, which are bound to List asynchronous event receiver as Remote event receiver.  Whole functionality is based on this article https://paulryan.com.au/2014/spo-remote-authentication-rest/. Therefore Paul Ryan deserves big thanks for this.
Keywords: SharePoint|online|REST|client|Authentication|Credentials|Username|Password