The easiest way to do this is with a JavaScript extension. The URL is in the global popclip.context.browserUrl
.
So, for example:
// #popclip
// name: Tot with URL
// language: javascript
let text = `${popclip.input.text}\n${popclip.context.browserUrl}`;
popclip.openUrl("tot://1/append?text=" + text);
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)