I am able to set slack reminder using /remind #public-channel but for private channel there is no option.
Found a solution for this. In case of a private-channel you can use slack inbuilt /remind command in following way:
/remind @channel every weekday at 2pm to start daily standup.
Here you can use either @here (to remind only online people) or @channel (remind everyone in the channel irrespective they are online).
This was a nifty trick I used with my private slack channel. Wont find it on slack documentation but it works.
I was able to set the reminder for the private channel by treating the name as a public channel as such :
Channel name : @private-channel
/remind #private-channel at 9AM every weekday to Start standup
Using #private-channel did not show the channel as an option in the list, but after submitting the reminder, it works