title: discord.py commands.when_mentioned not working with custom prefix
body: <p><strong>What I'm trying to do</strong>: If someone doesn't know the prefix, they can mention the bot and use the mention instead. After some research, I had found <a href="https://stackoverflow.com/questions/65379310/how-to-send-commands-to-a-bot-by-name">How to send commands to a bot by name?</a> This made me want to try using the <code>commands.when_mentioned</code> or <code>commands.when_mentioned_or</code> functions alongside my custom prefix.</p>
  <p><strong>My problem</strong>: The bot either only responds to the mention (while throwing errors at me), or does not respond at all.</p>
  <p>Here is the custom prefix code I am using: <a href="https://stackoverflow.com/questions/64513680/how-to-get-a-customizable-prefix-discord-py/64513681#64513681">How to get a customizable prefix discord.py</a></p>
  <p>Here is the client definition with the <code>command_prefix</code>:</p>
  <pre class="lang-py prettyprint-override"><code>intents = discord.Intents.all()
  client = commands.Bot(
      command_prefix= (get_prefix),
      description='A bot who wants your toes',
      owner_id=(394506589350002688),
      case_insensitive=True,
      intents=intents
      )
  </code></pre>
  <p><strong>Below I have listed what I have tried. I am not sure what to try next, so I will be grateful for any help provided.</strong></p>
  <p>Trial 1:</p>
  <pre class="lang-py prettyprint-override"><code>command_prefix= commands.when_mentioned_or((get_prefix))
  </code></pre>
  <p>Result:</p>
  <pre><code>TypeError: Iterable command_prefix or list returned from get_prefix must contain only strings, not function
  </code></pre>
  <p>Trial 2:</p>
  <pre class="lang-py prettyprint-override"><code>command_prefix= commands.when_mentioned or (get_prefix)
  </code></pre>
  <p>Result: No error, but bot no longer responds to custom prefix as seen below.</p>
  <p><a href="https://i.stack.imgur.com/1IQNO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1IQNO.png" alt="Bot does not respond to bl!" /></a></p>
  <p>Trial 3:</p>
  <pre class="lang-py prettyprint-override"><code>command_prefix= commands.when_mentioned and (get_prefix)
  </code></pre>
  <p>Result: No error, but bot no longer responds to mention as seen below.</p>
  <p><a href="https://i.stack.imgur.com/VAhc8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VAhc8.png" alt="Bot no longer responds to mention" /></a></p>