NodeRed -> Slack Bot issue, Slack not taking ngrok URL

I’m using HA in a docker, with NR at the default port 1880. I already have a functioning Slack bot and I simply use the call service node to call notify.slack_bot, and it works fine. What I want now is to be able to use NR to have the bot respond to messages.

I was trying out this NR chatbot package and I followed the guide down to setting up interactions/events (same instructions). I have generated my 8 hour ngrok URL at https://[magic numbers] and passed it as https://[magic numbers], but Slack gives me “Your request URL responded with an HTTP error. Update your URL to receive a new request and challenge value.” I also tried it as just http (no s).

I seem to be stuck.