Custom webhook responses?

Hi lovely NoCode folks, I was wondering if anyone had a good experience with a tool that allows custom webhook responses? For context, I'm trying to set up a Slack events call (https://api.slack.com/apis/connections/events-api) - the initial validation requires a response that Bubble, IFTTT, Zapier, Integromat, and Automate all seem to be unable to manage.

Any tips? I'd happily explore slightly more "code-y" options.