This actually works for me 👍. I created a personal access token with "repo" scope, put it in an environment variable as "secret", and then do the tagging and pushing in ci_post_xcodebuild.sh :
BUILD_TAG=b${CI_BUILD_NUMBER} git tag $BUILD_TAG git push --tags https://${GIT_PAT}@github.com/<User Name>/<Your Repository>.git
Topic:
Developer Tools & Services
SubTopic:
Xcode Cloud
Tags: