Custom variables for Mailgun

mailgun-1.rb

# Attempt 1:
# Send variables using v:[my variable]

api_key = "key-zxy" # my key
api_url = "https://api.mailgun.net/v2/drip.mailgun.org"

# Using the Faraday gem
connection = Faraday.new(:url => api_url) do |builder|
  builder.request :url_encoded
  builder.response :json, :content_type => /\bjson$/
  builder.adapter Faraday.default_adapter
end

connection.basic_auth("api", api_key)

data = {}
data["from"] = "Sender <sender@example.com>"
data["to"] = "recipient@example.com"
data["subject"] = "Subject"
data["text"] = "Text body"
data["html"] = "HTML body"
data["o:tracking"] = true
data["o:tracking-clicks"] = true
data["o:tracking-opens"] = true

# These are my custom variables I need to send with messages
data["v:delivery_id"] = 123
data["v:account_id"] = 456

connection.post do |req|
  req.url "messages"
  req.params = data
end

# The message gets sent, but custom variables do not appear in the webhook

mailgun-2.rb

# Attempt 2:
# Serialize variables into one param

api_key = "key-zxy" # my key
api_url = "https://api.mailgun.net/v2/drip.mailgun.org"

# Using the Faraday gem
connection = Faraday.new(:url => api_url) do |builder|
  builder.request :url_encoded
  builder.response :json, :content_type => /\bjson$/
  builder.adapter Faraday.default_adapter
end

connection.basic_auth("api", api_key)

data = {}
data["from"] = "Sender <sender@example.com>"
data["to"] = "recipient@example.com"
data["subject"] = "Subject"
data["text"] = "Text body"
data["html"] = "HTML body"
data["o:tracking"] = true
data["o:tracking-clicks"] = true
data["o:tracking-opens"] = true

# These are my custom variables I need to send with messages
data["v:my-vars"] = MultiJson.dump({
  "account_id" => 123,
  "delivery_id" => 456
})

connection.post do |req|
  req.url "messages"
  req.params = data
end

# The message gets sent, but custom variables do not appear in the webhook

mailgun-3.rb

# Attempt 3:
# Set serialized variables as a header

api_key = "key-zxy" # my key
api_url = "https://api.mailgun.net/v2/drip.mailgun.org"

# Using the Faraday gem
connection = Faraday.new(:url => api_url) do |builder|
  builder.request :url_encoded
  builder.response :json, :content_type => /\bjson$/
  builder.adapter Faraday.default_adapter
end

connection.basic_auth("api", api_key)

data = {}
data["from"] = "Sender <sender@example.com>"
data["to"] = "recipient@example.com"
data["subject"] = "Subject"
data["text"] = "Text body"
data["html"] = "HTML body"
data["o:tracking"] = true
data["o:tracking-clicks"] = true
data["o:tracking-opens"] = true

# These are my custom variables I need to send with messages
data["h:X-Mailgun-Variables"] = MultiJson.dump({
  "account_id" => 123,
  "delivery_id" => 456
})

connection.post do |req|
  req.url "messages"
  req.params = data
end

# The message gets sent, but custom variables do not appear in the webhook

it doesn't seem to be possible...
... at least not so far ...