Add functionality to "Create Issue" dialog

Hi,

I would like to hide/show fields in the dialog, based on the user selection.

Is it possible to add code (JS) to the Jira could OOB create issue dialog? and how

Thanks

In Jira Cloud it isn’t possible to extend the ‘create issue’ dialog, you can only extend the User Interface in specific defined locations (https://developer.atlassian.com/cloud/jira/platform/extending-the-user-interface-with-connect/).
There is a feature request for this functionality here.