Post proactive messages in Slack conversations with Bolt for Java

Question

I'm trying to start a proactive conversation with a user using the Slack Bolt SDK for Java but I don't know how to get a client for the Bot token, this is my code so far:

Bot bot = ...; // with botToken, botUserId, botId and so...

ConversationsOpenResponse conversation = app.client().conversationsOpen(conv -> conv
  .users(List.of(user.getSlackUserId()))
);

But every request I try to make returns with error=invalid_auth because it's laking the auth token (bot token in this particular case).

Every example I found relies on a context with a client tied to the installer bot.