curl --request PUT \
  --url https://gogs.example.com/api/v1/repos/{owner}/{repo}/issues/{index}/labels \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "labels": [
    123
  ]
}
'

[
  {
    "id": 123,
    "name": "<string>",
    "color": "<string>",
    "url": "<string>"
  }
]