I checked the source code and it is actually just a config file, so can be a snippet (slightly modified here):
// # popclip Logseq quick capture
// name: Logseq
// icon: iconify:simple-icons:logseq
// type: string
// captureHtml: true
// language: javascript
popclip.openUrl(`logseq://x-callback-url/quickCapture?content=${encodeURIComponent(popclip.input.markdown)}&url=${encodeURIComponent(popclip.context.browserUrl)}&title=${encodeURIComponent(popclip.context.browserTitle)}`)
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)
I hope you can use that as a basis for any modifications you need. Can be saved as a .popcliptxt of course.