解决方案

@result = RestClient.post(
          ‘url‘,
          {:billingSourceCode => "code"},
          {:headers => {‘Content-Type‘ =>‘application/json‘, 
                       "Authorization" => "key",  
                       "Accept" => "application/json"},
                       {:cookies => {:session_id => "1234"}}
          })

The params for the email is not in quotes. {"email": email@example.com, "password": password}. 

hHeader = {"X-AppSecretToken" => ‘tokenID1_sanitized‘, "X-AgreementGrantToken" => ‘tokenID2_sanitized‘, "Content-Type" => ‘application/json‘}

hCustomer = RestClient.get("https://restapi.e-conomic.com/customers/5", hHeader) # => creates a response showing customer 5 (shown for example of GET)

body = {‘address‘ => ‘Example Street‘, ‘name‘ => ‘John Doe‘}.to_json
RestClient.post "https://restapi.e-conomic.com/customers/", body, hHeader)