Templates developer have the option to add his own Javascript to make the template more active, the best way is to add Javascript code is to use the plain text editor.

In Templates manager click on "edit code" button instead "edit" button, this will open a plain text editor where you can write your own code.

 

This way will be suitable too If you want to add UTube, PayPal or Google Ads codes.

Remember, If you used the 'Text editor' to manage javascript once, NEVER use the WYSIWYG editor again to save the template or it will remove your Javascript.