Here is a full example which may help also referenced from the same page shared previously.

from symphony.bdk.core.activity.command import CommandContext
from symphony.bdk.core.config.loader import BdkConfigLoader
from symphony.bdk.core.symphony_bdk import SymphonyBdk


async def run():
    async with SymphonyBdk(BdkConfigLoader.load_from_symphony_dir("config.yaml")) as bdk:
        activities = bdk.activities()
        messages = bdk.messages()

        @activities.slash("/buy {$ticker} {quantity}")
        async def on_echo_mention(context: CommandContext):
            ticker = context.arguments.get_cashtag("ticker").value # can also be retrieved with context.arguments.get("ticker").value
            quantity = context.arguments.get_string("quantity")
            
            message = f"Buy ticker {ticker} with quantity {quantity}"

            await messages.send_message(context.stream_id, f"<messageML>{message}</messageML>")