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.
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.
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.
Insert the script that starts and ends with <script></script> and save the page.