Attempting to use keybase.io API with PowerShell

Keybase-IO.ps1

<#
  .SYNOPSIS
  Keybase experiment.
  .DESCRIPTION
  Beginnings of a PowerShell script to post a Keybase signature announcement. Still not finished but the following commands do work, which is nice.
  .PARAMETER SignedJsonPath
  A file containing the signed JSON Object, e.g. created using the following command,
    gpg2 -u "..." -a --sign keybase-json.txt
  .PARAMETER Session
  .PARAMETER CsrfToken
  .PARAMETER SigningKid
  .PARAMETER Uid
  .EXAMPLE
  Keybase-IO -SignedJson .\keybase-json.txt.asc
  .LINK
  tbc
#>
param(
  [Parameter(Mandatory=$true)][string]$SignedJsonPath,
  [Parameter(Mandatory=$true)][string]$Session,
  [Parameter(Mandatory=$true)][string]$CsrfToken,
  [Parameter(Mandatory=$true)][string]$SigningKid,
  [Parameter(Mandatory=$true)][string]$Uid
)

$url = "https://keybase.io/_/api/1.0/follow.json"

$in = Get-Content -Path $SignedJsonPath -Raw

Write-Host "Posting signature:"
$in.substring(0,200)

$sig = $in -replace '\r',''

$headers = @{
    "X-CSRF-Token" = $CsrfToken
}

$body = @{
    sig = $sig
    type = "track"
    session = $Session
    plain_out = "1"
    signing_kid = $SigningKid
    uid = $Uid
}

Invoke-RestMethod -Method Post -Uri $url -Headers $headers -Body $body