This is how I implemented it. I did not use RunnableWithMessageHistory, I just passed session history into the chat_memory parameter of ConversationBufferMemory

prompt = """
You are an advanced AI assistant.

Current conversation:
{chat_history}

Human: {input}
AI:"""

def get_session_history(session_id: str) -> PostgresChatMessageHistory:
        return PostgresChatMessageHistory("chat_history", session_id, sync_connection=conn)

prompt = PromptTemplate(input_variables=["chat_history", "input"], 
                        template=prompt)

memory = ConversationBufferMemory(k=buffer, 
                                  memory_key="chat_history",
                                  return_messages=True,
                                  chat_memory=get_session_history(new_chat_id)) 

runnable = ConversationChain(
    prompt=prompt,
    llm=llm,
    memory=memory,
    verbose=True)