Hy, 

I try to post a message (inside the chatter) using External API into an opportunity. 

I tried this code : 

response=models.execute_kw(db, uid, password, 'mail.message', 'create', [{ 'model': 'crm.lead', 'res_id': 27, 'body': "test"}])
print(response)

But I only get as answer : "1704"

When I look at the opportunity, there is nothing displayed in the chatter. 

Anyone can help me ?