response = client.chat.completions.create(
  model="gpt-4o-mini",
  messages=[
   {"role": "user", 
   "content": "Please write down four trees with their scientific names in json."}
 ],
  response_format={"type": "json_object"}
)
print(response.choices[0].message.content)

{"trees": [{"common_name": "Oak", "scientific_name": "Quercus"}, { "common_name": 
"Maple", "scientific_name": "Acer"}, {"common_name": "Pine", "scientific_name": 
"Pinus"}, {"common_name": "Birch", "scientific_name": "Betula"}]}