PowerBI vs ARIS API

I'm investigating PowerBI reports vs the ARIS Connect REST API in order to present key results for our model numbers, freshness, group hierarchy, model types &c. However, I'm getting stuck on generating and using the session key to do the actual REST queries. PostMan works fine with a POST query like http://aris.myorganization.com/umc/api/v1/tokens?tenant=default&name=admin&password=topsecret, but PowerBI Desktop keeps insisting that the query must be anonymous in order to POST. 

Could anyone share a working skeleton PowerBI report that works to retrieve any data? I can do manual refresh of the session keys if I absolutely must, but won't be able to publish reports for others that way. 

Added

I'm trying to setup an empty datasource with an M query as below, but PowerBI Desktop claims authentication fails, with no more details. I've tried clearing out file and global authentication settings for the data source, but the global settings for http://<arisserverURL>/umc/api/v2/tokens refuses to clear. Time to hit the PowerBI forums, I guess.

let
    ArisDatabaseURL = "http://<arisserverURL>/",
    KeyRequest = Json.FromValue([tenant="default", name="system", password="*******"]),
    KeyResponse = Web.Contents(ArisDatabaseURL & "umc/api/v2/tokens",
        [Content=KeyRequest]),
    KeyDoc = Json.Document(KeyResponse),
    ApiKey = KeyDoc{0}[token],
    Cookies = "accesstoken=" & ApiKey,
    Databases = Web.Contents(ArisDatabaseURL & "abs/databases", [Headers = [ Cookie = "accesstoken=" & Cookies]] )
in
    Databases