Using Get channel details option, we can easily get the Slack Channel Id.

We can use the following steps How to get the Slack Channel Id:

1. Open Slack App or open the Slack from your browser.

2. Click the Channel on the left navigation pane.

3. Click “Get channel details”. The Channel ID will be in the bottom.

When we use “Send Slack Message” Salesforce Flow element to send a Slack Message, Slack Conversation ID should be updated with the Channel ID to send the Slack Message from the Salesforce Flow.