createsend.rb

 body = {
       grant_type: 'authorization_code',
       client_id:  Rails.application.secrets[provider]['client_id'],
       client_secret:  Rails.application.secrets[provider]['client_secret'],
       redirect_uri: integration_callback_url(provider),
       code: params[:code]
       }.to_query
       uri = URI.parse('https://api.createsend.com/oauth/token')
       http = Net::HTTP.new(uri.host, uri.port)
       http.use_ssl = true
       request = Net::HTTP::Post.new(uri.request_uri)
       request.body =  body
       ActiveSupport::JSON.decode(http.request(request).body)['access_token']
       
       # returns error 400