You can add custom scripts to your WordPress websites using a plugin and perhaps it's the best and safest way if you trust the plugin. When you don't need the scripts, you can take off them and disable the plugin. There are few plugins in WordPress repository but none of them using Customizer feature. So you need to add them from WordPress Dashboard, save script and check back the pages to test the functionality. Since I often need to add some scripts to add interactivity or custom features, I felt it should use the Customizer so that it could be tested on the fly. So I built a fresh new plugin for this and released for free on the WordPress repository. You can check Custom Header Footer Scripts for Customizer
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