curl --request POST \
  --url https://api.example.com/api/v1/workspaces/{workspace_id}/billing/seats \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '
{
  "additional_seats": 123
}
'

{
  "extra_seats": 123,
  "seat_limit": 123
}