Hi there, could you tell us what is the trigger you are trying to achieve ? is it not in the Slack piece already ?
If you are trying to setup Slack webhooks, I would recommend you to go with self hosting and reading this in our docs: Setup App Webhooks - Activepieces
But just in case, can you paste the webhook url in your browser with /sync added to the end of it make sure the output is like you need? and then try again with Slack ?