That’s not the workflow I use.
- Sync project to Mac
- Go to developer.apple.com and create all of the certificates and provisioning profiles, add your devices, etc. Create both development and distribution profiles.
- Go to XCode preferences->accounts and log in to your developer account. Click “Download manual profiles”.
- Go to iTunes Connect and create your application using your Bundle ID
- Load UE and select the distribution certs for signing. Set your app to “shipping” and “distribution”. Explicitly set your bundle ID (com.mygame.whatever). Build it.
- Load up the Application Loader program on your Mac (maybe via XCode) and upload your .ipa file.
Once you’ve uploaded it to Testflight, it’ll go through a review process the first time. At this point you can add individual testers (get them to make an iTunes Connect account) or you can create external test groups. External test groups require ANOTHER round of review the first time and it takes longer, but it’s nice in that you can choose to release builds to them or not even if they’re uploaded to Testflight.
At no point do you need to do anything else in XCode. You don’t even need your project open.
The main thing I use XCode for besides downloading the certs is to deploy already-built dev builds to the device, since the launch button doesn’t deploy everything.