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The Issue Server URL is a template URL for generating links to your issue system. A simple way to construct the URL is to take a link to a certain issue in your issue system then substitute the issue id field in it with {id}. When the issue id's are parsed out of the commit message using the commit message regex, they'll then be substituted into the template URL where the {id} field was placed.
After inputting the regex and URL you can verify your settings by providing a typical commit message you would use when checking in a fix for a bug. After clicking "Verify", and if it was successful, you should see a hyperlink that will take you to that issue in your issue tracker.
Link an Issue Connector to a Repository
Once an Issue Connector has been created and for it to be any use, it must be linked to a repository. To do this, select edit on the Repository you wish to add an issue connector to and select the Issue Connector you created from the drop down. Save the Repository and from then on the Issue Connector will monitor repository checks and try parse issue id's out of them.