My company just built an app for a client who already has an Apple Developer account and had already created their Distribution Certificates. When the cert is downloaded to our
Please ask client to provide you Certificate.p12 that will include the certificate and the private key.
In order to create .p12, go to Keychain Access, select the certificates option in second block in left pane, and then select the certificate you want and right click > Export > Enter the password for p12 > enter Mac password > save at your desired location.
To install at your side, just get that p12 from client, double click to open it, and enter the password which client has entered for p12.
Finally, you are done with getting distribution certificate which is signed at clients place.
So I've addressed this one of two ways with my clients. Assumption here is that the client trusts you enough...
Ask for the client's Apple Developer login and password info, revoke the current distribution certificate and build a new one using your build machine. Boom! Sign and submit.
Ask the client to supply you with the private key he/she used to sign the original distribution certificate. This is done via Keychain Access (see Ronak's answer for specifics).
If said client gets leery about #1, he/she can change their development account password once the app gets submitted and approved, then revoke/make a new distribution certificate.
Getting your client's private key (#2) poses some security risks for your client, but it's the only other way I'm aware of.
you need to create one in your portal and generate one for your distribution to app store.