First, go to the Harness Admin Dashboard to COPY your code + customize your Floating Button:
Copy the code on the Harness "Setting > Website Code" page listed above and then go to your WordPress Admin Dashboard
Click on "Pages" in the left sidebar menu
You're going to have to add the code to each page individually (if your organization had the plugin option it only needs to be done once, but based on the permissions/setup, it'll need to be added to each page)
Hover over a page and click on the "Edit"
In the "Editor" the top right corner will have 2 tabs for "Visual" and "Text" - Click on the "Text" version
Now all you have to do is scroll to the bottom of the page and PASTE the code you copied from the Admin Dashboard into the page editor
Then click on "Update" (or preview if you'd like to see it)
The code does not display (it's invisible if you're using the "Text" tab) but it allows the button to display. Since you'll have to do it on all pages individually, I'd start with your "Homepage" and "Donate" page so they're immediately added.