On a Mac or PC, it's easy to find the alphanumeric ID of a Slack channel.
Step 1. Enter your Slack team URL in your web browser
The URL follows the pattern yourteamname.slack.com, but replace “yourteamname” with your actual team name.
If you are not yet signed in to your team, sign in now
Step 2. Click on the channel
The channels are displayed under the header Chains in the left column
Step 3. Locate the channel ID
This can be found in the address bar between / messages / and / team /. The address bar is the one at the top of your browser that displays the URL of the active website. The Channel ID is nine characters long and can contain both letters and numbers.