Oauth 2.0: Session API Token Request Fails with Special Chrs in Body?!? (Rest API)

Hi there,

First... Thank you all for the earlier help!  What a great community!  I'm so close here...  Nevertheless, I'm continuing to struggle with one last bit in obtaining a "session" token from an API within Power Query.  I had a post earlier about it, (thanks to artemus for a start), but to save others the hassle, where we landed I wanted to summarize it in a new thread (I'll link back to this one). 

I highly "suspect" the problem might do with the special values embedded in the string.  In the code below, VALUE1 contains "!" (i.e. abc123!) and VALUE3 contains "+" and "=" (i.e. abc+123=).  I can't get ride of either of these.  If it isn't the special characters, it is something like a missing character or flag.

Any thoughts would be much appreicated!  I just got dumped this in my lap and am trying to learn and address it at the same time.

RefreshAPIToken = () as text =>
        let token_url = "https://DOMAIN/api/Token",
    body="Username=VALUE1&Password=VALUE2&grant_type=password&Integrated=N&database=VALUE3&Client_Id=VALUE4=&client_secret=VALUE5", 

params =
        [ 
            Headers = [#"Content-Type"="application/x-www-form-urlencoded", #"Accept"="*/*", #"Accept-Encoding"="gzip, deflate"],
            Content = Text.ToBinary(body)
        ],
    WebSource = Json.Document(Web.Contents(token_url,params)),
    apikey = WebSource[access_token]
   
    in
        apikey

PLEASE NOTE: Tried Text.AsciiEncoding and BuildQueryString using same string of text.  Also added a few parameters to ToBinary (such as Base64 and Compress).

The code runs fine in PostMan, and if I retrieve the session token from PostMan, I can plug it in and pull my data in Power BI.  I'm getting a 400 reply error, but I am not sure how to debug what is being sent by Power BI, nor the specific error message details being received.  If I don't add "Text.ToBinary", Power BI fails saying it can't convert to binary.

According to PostMan I need to do Oauth 2.0 and "Request Headers" (as opposed to URL") in authorization data.  I'm not sure where or if I need to setup either of these within Power BI.  Power BI is setup for an anonymous connection.