Here are some free webhook icons from the OpenWeb Icons Set. These are great to represent your webhook feature in your app or as graphics for blog posts/other content. The black and white icon is solid, which makes it easy to change/add colors. They are available under the SIL Open Font License 1.1.
If you're building a webhook solution, consider leveraging our webhooks as a service offering to build a secure, reliable, and scalable webhook solution. We have an open source library as well a paid hosted solution with a generous free tier:
Black and White