Converting Postman Request into PowerBI

Given the JSON string you defined as your header content, I first converted it to a record in Power Query and then into a binary (JSON) format using the `Json.FromValue` function.

Even though I haven't tested it, this should get you on your way:

let
    username = "",
    password = "",
    accountcode  = "",
    bodyContent = [
        select = {
            "mailing_name",
            {
                "MAX('stats_date')",
                "stats_date"
            },
            "sum('sent')",
            "sum('success')",
            "sum('failed')",
            "sum('opens')",
            "sum('unique_opens')",
            "sum('unsubscribes')",
            "sum('complaints')",
            "sum('clicks')",
            "sum('unique_clicks')",
            "mailing_id",
            "list_id",
            "from_address",
            "email_message_subject",
            "schedule_date"
        },
        from = "mailing",
        group = {
            "list_id",
            "mailing_id"
        },
        list_ids = "all",
        get_extra_conversion_points = true,
        filter = {
            {
                "stats_date",
                ">=",
                "2020-01-01"
            },
            {
                "stats_date",
                "<=",
                "2020-12-30"
            }
        }
    ],
    binaryContent = Json.FromValue(bodyContent),
    request = Web.Contents(
        "api.ongage.net",
        [
            Headers = [
                #"X_USERNAME" = username,
                #"X_PASSWORD" = password,
                #"X_ACCOUNT_CODE" = accountcode,
                #"Content-Type" = "text/plain"
            ],
            RelativePath = "/api/reports/query",
            Query = [],
            Content = binaryContent
        ]
    )
in
    request