You can call any Web API method using the WebClient provided to your Bolt app as either app.client or client in middleware/listener arguments (given that your app has the appropriate scopes). When you call one the client's methods, it returns a SlackResponse which contains the response from Slack.

The token used to initialize Bolt can be found in the context object, which is required to call most Web API methods.

@app.message("wake me up")
def say_hello(client, message):
    # Unix Epoch time for September 30, 2020 11:59:59 PM
    when_september_ends = 1601510399
    channel_id = message["channel"]
    client.chat_scheduleMessage(
        channel=channel_id,
        post_at=when_september_ends,
        text="Summer has come and passed"
    )