To add a web widget to a Squarespace site you need to create your web widget in the normal way and add it to your Squarespace site in a code block
Paste the widget HTML into your code block
Add the following lines to the top of your code block before the widget html
You may need to test the widget using your own domain name rather than the default Squarespace domain - e.g. Squarespace may give you a default domain like www.mycharity.squarespace.com - you would need to connect up your own domain to your Squarespace site - e.g. www.mycharity.org - and test using that.
Your web widget will then work in Squarespace.
To get the appearance of the web widget exactly as you want it you can add your own CSS - this article explains more.