You can add Jira Server Alerts in Slack by going to the channel you want to use, then Conversation Settings> Add Apps > Search Jira Server Alerts (legacy) add that.
Then it should give you instructions on what you need to do in Jira. You will need admin access so you can go System > WebHooks > Create WebHook and then you go through and add it in.
There is some reading on it here; https://confluence.atlassian.com/adminjiraserver073/managing-webhooks-867028229.html
Should be enough to get you going :)
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