```ruby
require 'httparty'
require 'singleton'
require 'ostruct'

module EhjobAuthentication
  class ApiClient
    include Singleton
    include HTTParty

    format :json
    default_options.merge!(verify: false)
    headers 'Content-Type' => 'application/json'
    headers 'Accept' => 'application/json'
    debug_output $stderr if Figaro.env.httparty_debug?

    headers 'Authorization' => "Token token='#{Figaro.env.single_authentication_key}'"

    delegate :base_url, to: 'EhjobAuthentication.config'

    def associate_user(params)
      url = "#{base_url}/api/users/associate_user"
      response = self.class.post url, body: JSON.generate(params)
      OpenStruct.new(response.parsed_response) if response.success?
    end
  end
end
```

Rename the authorization key in request headers
user
```ruby
require 'httparty'
require 'singleton'
require 'ostruct'

module EhjobAuthentication
  class ApiClient
    include Singleton
    include HTTParty

    format :json
    default_options.merge!(verify: false)
    headers 'Content-Type' => 'application/json'
    headers 'Accept' => 'application/json'
    debug_output $stderr if Figaro.env.httparty_debug?

    headers 'HTTP_AUTHORIZATION' => "Token token='#{Figaro.env.single_authentication_key}'"

    delegate :base_url, to: 'EhjobAuthentication.config'

    def associate_user(params)
      url = "#{base_url}/api/users/associate_user"
      response = self.class.post url, body: JSON.generate(params)
      OpenStruct.new(response.parsed_response) if response.success?
    end
  end
end
```
assistant