FTX.com REST API POST Authentication FAILS with Ruby on Rails and net/https

Hoping for some help as this one has me baffled...

I created a user account and API credentials at FTX.com.

They have an interesting Auth setup which is detailed here: xhxtxtxpxsx://docs.ftx.com/?python#authentication

They only provide code examples for python, javascript and c#, but I need to implement the integration on a RoR app.

Here's a link which also provides an example for both GET and POST calls: xhxtxtxpxsx://blog.ftx.com/blog/api-authentication/

I'm using:

ruby '3.0.1'

gem 'rails', '~> 6.1.4', '>= 6.1.4.1'

also,

require 'uri'
require 'net/https'
require 'net/http'
require 'json'

I got the authentication working for GET calls as follows:

def get_market
 get_market_url = 'xhxtxtxpxs://ftx.com/api/markets/BTC-PERP/orderbook?depth=20'

 api_get_call(get_market_url)
end

def api_get_call(url)
    ts = (Time.now.to_f * 1000).to_i

    signature_payload = "#{ts}GET/api/markets"

    key = ENV['FTX_API_SECRET']
    data = signature_payload
    digest = OpenSSL::Digest.new('sha256')

    signature = OpenSSL::HMAC.hexdigest(digest, key, data)

    headers = {
      'FTX-KEY': ENV['FTX_API_KEY'],
      'FTX-SIGN': signature,
      'FTX-TS': ts.to_s
    }
    uri = URI.parse(url)
    http = Net::HTTP.new(uri.host, uri.port)
    http.read_timeout = 1200
    http.use_ssl = true
    rsp = http.get(uri, headers)
    JSON.parse(rsp.body)
  end

This works great and I get the correct response:

=>
{"success"=>true,
"result"=>
{"bids"=>
[[64326.0, 2.0309],
...
[64303.0, 3.1067]],
"asks"=>
[[64327.0, 4.647],
...
[64352.0, 0.01]]}}

However, I can't seem to authenticate correctly for POST calls (even though as far as I can tell I am following the instructions correctly). I use the following:

  def create_subaccount
    create_subaccount_url = 'xhxtxtxpxsx://ftx.com/api/subaccounts'

    call_body =
      {
        "nickname": "sub2",
      }.to_json

    api_post_call(create_subaccount_url, call_body)
  end

  def api_post_call(url, body)
    ts = (Time.now.to_f * 1000).to_i

    signature_payload = "#{ts}POST/api/subaccounts#{body}"

    key = ENV['FTX_API_SECRET']
    data = signature_payload
    digest = OpenSSL::Digest.new('sha256')

    signature = OpenSSL::HMAC.hexdigest(digest, key, data)

    headers = {
      'FTX-KEY': ENV['FTX_API_KEY'],
      'FTX-SIGN': signature,
      'FTX-TS': ts.to_s
    }

    uri = URI.parse(url)
    https = Net::HTTP.new(uri.host, uri.port)
    https.use_ssl = true
    request = Net::HTTP::Post.new(uri, headers)
    request.body = body
    response = https.request(request)

    JSON.parse(response.body)
  end

This is the error response:
=> {"success"=>false, "error"=>"Not logged in: Invalid signature"}

My feeling is the issue is somewhere in adding the body to signature_payload before generating the signature via HMAC here..?:
signature_payload = "#{ts}POST/api/subaccounts#{body}"

Thinking this because, if I leave out #{body} here, like so:
signature_payload = "#{ts}POST/api/subaccounts"
the response is:
=> {"success"=>false, "error"=>"Missing parameter nickname"}

I have tried several iterations of setting up the POST call method using various different net/https examples but have had no luck...
I have also contacted FTX support but have had no response.

Would truly appreciate if anyone has some insight on what I am doing wrong here?