Find this in Slack: click the person's profile → "More" (•••) → "Copy member ID". "Look up" auto-fills name/email below.
Slack settings
How do I find a Channel ID?
In Slack, open the channel → click the channel name at the top →
scroll to the bottom of "About" → the Channel ID is listed there
(starts with C). Make sure the bot has been invited to
this channel with /invite @YourBotName, or the topic
update will silently fail.