Install Wordpress Plugin: "Simple Custom CSS and JS"

Once Installed and Activated, you can find an extra option "Custom CSS & JS" appear on the left hand side menu bar:

Add Custom HTML Code

Click "Add HTML Code" to add a new code block:

Name the block as "CoTreat WebChat", and copy and paste the code from the CoTreat Practice WebChat Setting:

Click "Publish" on the right hand side, the widget should appear on your Wordpress website now.

Want to know how to use Web Chat? How to use Web Chat ? - CoTreat Help Center (helpscoutdocs.com)