What is the purpose of lock to this channel when configuring an incoming webhook?
It means that if it’s enabled, then the incoming webhook can only post to the channel that you selected.
Looking at our documentation it seems that you can only post to one channel at a time, so this would be a good Feature Request: https://mattermost.uservoice.com/forums/306457-general.
A workaround would be to use slash commands which can be used in multiple channels.
I have submitted the feature request. I hope we can upgrade soon. Thank you for your time and advice.