The easiest way to do this is with a workflow.
Make a workflow triggered by “when someone joins a channel.” Make that channel selection as your #welcome/#general channel. This is a special channel that everyone gets added to automatically when their Slack account is created.
That way when they log into Slack, they’ll have a one-time message from your workflow with the list of action items.
It doesn't seem possible to do so as an Admin @Host Ever or it's because we're on Pro plan and not on Business/Entreprise Grid.
But are we 100% sure we can customize banners? From this help article, I'm unsure if it's just for telling Guests/Users who can see their messages, or if we can leverage it as we want
Similar question asked here: https://forums.slackcommunity.com/s/question/0D5PU00000hNh8d0AC/can-a-message-banner-be-implemented-on-channels?language=en_US
