slack chat.postMessage API endpoint is not allowing the authorization header

Solution 1:

I don't understand, I need to specify the bearer token somehow, even in the docs it says to put it in the Authorization header, why aren't they allowing it?

This is a different problem, is not related to the Bearer token at all. From the error you're getting, it means, the origin you're using to fetch the Slack API, is not trusted (http://127.0.0.1:5500), there is nothing you can do from the browser since this is a policy that comes from the server which defines the authorized origins. (Learn more about CORS here) Since I don't think this is supported by Slack, you will need to fetch the Slack API from the server.

One way to solve this, is by exposing a backend API, for example:

Post a message to Slack	Run in Fusebit

router.post('/api/tenant/:tenantId/test', async (ctx) => {
  // Create a Slack client pre-configured with credentials necessary to communicate with your tenant's Slack workspace.
  // For the Slack SDK documentation, see https://slack.dev/node-slack-sdk/web-api.
  const slackClient = await integration.tenant.getSdkByTenant(ctx, connectorName, ctx.params.tenantId);

  // Get the Slack user ID associated with your tenant
  const slackUserId = slackClient.fusebit.credentials.authed_user.id;

  // Send a Direct Message to the Slack user
  const result = await slackClient.chat.postMessage({
    text: 'Hello world!',
    channel: slackUserId,
  });

  console.log('message response', result.message);
  ctx.body = { message: `Successfully sent a message to Slack user ${slackUserId}!` };
});