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The most "tricky" part will be to generate the authorization header for the request. Connec!™ webhooks use uses HTTP Basic authentication to connect to your application, using the API credentials that were provided to you directly or via the App Provider dashboard on Maestrano.
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Once done, just add the Authentication header to Advanced REST client:
Method and Content-Type
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If you have followed the instructions above, you're all setup! You can now fill the payload section with notifications and hit send!