We are trying to setup deep linking into slack, as described here:
https://api.slack.com/docs/deep-linking
The document states that to open a specific team,
The easiest way to get the team and channel id and is to add slack developer tool app to your slack, you can can it here.
After adding it all you simply need to do is go to any channel and choose this command
/sdt whoami
It will output a snippet like this one.
There are much more commands and other things you can perform with this slack app, I would highly recommend it if you're building any integrations for slack.
As of July 2019, it seems like the team id and the channel id is already being shown via the web.
As you can see in the screenshot, the 2nd to the last node is the team id (prefixed with "T") and the last node is the channel id (prefixed with "C"). All you have to do is open slack via the web, go to the specific channel, and you can see the url change to something close to the screenshot above.
You can also simply right-click on the slack channel (or direct message) in the desktop App and select the 'Copy Link' option.
This will give you a URL of the form: https://org.slack.com/messages/[channel-ID]
I attest this works on High Sierra Mac OSX - Slack 3.1.1