It would be good if you could paste SVG code icons directly
Ps: To be honest, I’ve never dared to try it before because the plugin’s website looks abandoned or outdated. It’s striking that the website isn’t responsive, especially when we’re talking about the website for a WordPress plugin 😉
You can use SVG code icons if you encode them the way that WordPress expects (see the $icon_url parameter). Specifically, base64-encode the icon, and add data:image/svg+xml;base64, to the beginning of the code. Then you can enter the result in the “Icon URL” field.
For example, let’s take this icon from Bootstrap Icons: