Cannot communicate to Azure Devops API through any authentication

I have an extension in Azure Devops that allows HTTP GET on https://{organisation}.extmgmt.visualstudio.com/_apis/ExtensionManagement/InstalledExtensions/timelog/time-logging-extension/Data/Scopes/Default/Current/Collections/TimeLogData/Documents/ and it returns JSON data. This works fine in browser but I can't get it to load using 'Web' data source in Power Query/BI. I'm stuck at the authentication page.

1. When I use 'Organizational Account' type authentication, the error I'm getting is, "The resource principal named https://{organisation}.extmgmt.visualstudio.com was not found in the tenant named {tenant-id}. This can happen if the application has not been installed by the administrator of the tenant or consented to by any user in the tenant. You might have sent your authentication request to the wrong tenant."
2. When I try to use Personal Access Token (PAT) in Azure (with full access), I couldn't get through the authentication page! This is the code I tried is something like:

let
Source = Web.Contents("https://{org}.extmgmt.visualstudio.com/_apis/ExtensionManagement/InstalledExtensions/timelog/time-logging-extension/Data/Scopes/Default/Current/Collections/TimeLogData/Documents/", [Headers=[Accept="application/json", Authorization="Basic cXZvMjNhNHhneGNza29kN203NTQ1cTBuFczbWdvbGlhdXFxYWpnMzVqa2hnZjNlc2FmcQ=="]]),
Result = Json.Document(Source)
in
Result

Can you advise what kind of authentication mechanism (and how it) can be used to get this resultset through programs like PowerQuery editor? I just want the output JSON to become a data source for Power BI.

Thanks!