Obviously, the simplest solution would be for the client to share their account details or add us as ‘team admin’, but that is not what this is about.
- Add us to your iOS Developer Program as ‘team member’.
- Create a ‘Distribution Certificate’, if you haven’t got one already.
- Create a ‘App Store Distribution Provisioning Profile’.
- Export the ‘Distribution Certificate’ assets and send the export and password to us. (For security sake, it’s a good idea to send us the password via other means than the exported certificate. E.g. by phone/SMS.)
- Ensure all relevant contracts have been signed.
- Create the application record in iTunes Connect. Ensure that the app record status in iTunes Connect is ‘Waiting for Upload’.
We upload a lot of apps for clients and even setting up the app record in iTunes Connect is too much. I suggest asking them for the specific assets and marketing information required and entering that yourself.
I believe @manishmalani is correct on the second point as well.
The easiest thing from the client's perspective is to ask for their agent credentials and do everything for them, and that's what I would suggest, unless they have a strong preference to upload the IPA themselves.