help with POST api call

I am trying to replicate the API call which includes refresh of an access token as described here: https://community.fabric.microsoft.com/t5/Power-Query/Refresh-token-api-call/td-p/937553 

My API uses POST with "body" text instead of parameters. I have confirmed the POST call with the correct BODY parameters works thru Postman, but I can't seem to get the formatting correct in Power Query and am just getting "(400): Bad Request" from the line that makes the Web.Contents call. 

Here is the PQ code

let
    //App Details
    client_id = #"client_id", //<===parameter
    client_secret = #"client_secret", //<===parameter
    
    //Authentication
    
    //URL's
    token_uri = #"token_url", //<===parameter
    register_url = #"register_url", //<===parameter

    //body = Text.Combine({"{","""login_id""",":", """", client_id, """", ",", """security_key""", ":", """", client_secret,"""","}"}),
    body = Json.FromValue([login_id = client_id, security_key = client_secret]),
    header = [#"Content-Type"="application/json"],
    tokenAsk = Web.Contents("https://accounts.pyrus.com/api/v4/auth", [Headers = header, Content = body]),
   tokenResponse = Json.Document(tokenAsk),
    access_token = tokenResponse[access_token],
    token = Text.Combine({"Bearer ", tokenResponse[access_token]})
// GetJsonQuery = Web.Contents("https://api.pyrus.com/v4/forms/515736/register", [Headers=[#"Authorization"="token"]]),
//FormatAsJsonQuery = Json.Document(GetJsonQuery)
 
in
tokenResponse

As you can see I tried "body" in 2 different ways, and neither has worked.

The code snippet from Postman that works is (login/pw masked)

curl --location 'https://accounts.pyrus.com/api/v4/auth' \
--header 'Content-Type: application/json' \
--header 'Cookie: www=kuberg1' \
--data-raw '{
    "login": "[email protected]",
    "security_Key": "******"
    }'

(I don't know what kuberg1 as a cookie is - that was the default and I'm generally not a web api coder)

Can anyone see what I'm missing in my PowerQuery script?

Thanks,

David