Use selected_group parameter

ee/app/helpers/subscriptions_helper.rb

@@ -19,7 +19,7 @@ module SubscriptionsHelper
   def addon_data(group)
     {
       group_data: [present_group(group)].to_json,
-      namespace_id: params[:namespace_id],
+      namespace_id: params[:selected_group],
       source: params[:source]
     }
   end

ee/spec/helpers/subscriptions_helper_spec.rb

@@ -141,7 +141,7 @@ RSpec.describe SubscriptionsHelper do
 
     before do
       allow(helper).to receive(:current_user).and_return(user)
-      allow(helper).to receive(:params).and_return({ namespace_id: group.id.to_s, source: 'some_source' })
+      allow(helper).to receive(:params).and_return({ selected_group: group.id.to_s, source: 'some_source' })
       group.add_owner(user)
     end