Adding third-party apps or javascript to your pages is very simple. To add scripts globally for your site or pages, click on Website Settings -> Custom Code.
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If you have a code that starts and ends with <script></script>, you have to add it in the Custom Head Markup or Page Head Markup.
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Click on the Save button at the bottom and preview your site to see the script on your page.
To add a script inside your page in a specific place, use the Code Widget from Add Widget -> drag and drop the Code Widget anywhere on your page.
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Insert the script that starts and ends with <script></script> and save the page.