You can use the "Manage head tags" feature to install third-party services onto your pages (see Integrations for the list of useful services we have compiled):
For example, if you want to add a chat widget, you can sign up with tawk.to for free and they will give you a widget code. Widget codes usually look like
<script> ... </script> with several lines of code in-between.
Paste the code into the "Manage head tags" dialog:
Use the "Apply to subpages" checkbox if you want the integration to be applied to all pages. Otherwise it will only be applied to a single page.