MDM Payload
I think it should be possible. Never tried it myself though.
Can I have information about the payloads available on iOS Client?
I don't have any information about it. You would probably need to do your own research.
where are the iOS app sources?
Have you tried clicking on the "source code" in the menu or at the bottom of this page?
can you review this request https://www.traccar.org/forums/topic/feature-request-mdm-settings-deployment/ and consider making those changes to be able to send the client config parameters thru a payload?
Can I configure the client app thru an MDM payload?