curl --request PUT \
  --url https://discord.com/api/v10/lobbies/{lobby_id}/members/{user_id} \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "metadata": {},
  "flags": "<unknown>"
}
'

{
  "id": "<string>",
  "metadata": {},
  "flags": 123
}