Method: cases.comments.create

 case="projects/some-project/cases/43591344"
curl \
  --request POST \
  --header "Authorization: Bearer $(gcloud auth print-access-token)" \
  --header 'Content-Type: application/json' \
  --data '{
    "body": "This is a test comment."
  }' \
  "https://cloudsupport.googleapis.com/v2/$case/comments" 

 import googleapiclient.discovery

apiVersion = "v2"
supportApiService = googleapiclient.discovery.build(
    serviceName="cloudsupport",
    version=apiVersion,
    discoveryServiceUrl=f"https://cloudsupport.googleapis.com/$discovery/rest?version={apiVersion}",
)
request = (
    supportApiService.cases()
    .comments()
    .create(
        parent="projects/some-project/cases/43595344",
        body={"body": "This is a test comment."},
    )
)
print(request.execute())