Once you've obtained the certificate and private key for iOS (you can follow this guide for that), you can create a new iOS push_credential with the following request:
curl -X "POST" "https://api.telnyx.com/v2/mobile_push_credentials" \
-H 'Authorization: Bearer <my-telnyx-api-key>' \
-H 'Content-Type: application/json; charset=utf-8' \
-d '{
"certificate": "<my-iOS-Certificate>",