Suggest to read the documents :
https://developer.atlassian.com/display/JIRADEV/JIRA+Webhooks+Overview
https://confluence.atlassian.com/display/JIRA/Managing+Webhooks
To complement Vishnu answer, a good way to test webhooks is using requestb.in as your webhook destination. Then you can see the json of what was sent and check if it's right.
Cheers
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