@client.event
async def on_message(message):
    botID = # Enter your bot's ID here
    if message.content == f'<@!{botID}>':
        with open('prefixes.json', 'r') as f:
            prefixes = json.load(f)
        prefix = prefixes[str(message.guild.id)]
        await message.channel.send(f'My prefix is: {prefix}')