Got it.
PopClip uses the default system browser except when being invoked from a different browser. In those case it stays in that browser.
You’ll need to force PopClip to use OpenIn even when invoking from a browser. We can take the approach as shown in the thread above. Like this:
// #popclip extension to search Forest Admin
// name: Forest Admin - Org
// icon: search filled for
// language: javascript
const query = encodeURIComponent(popclip.input.text);
const url = `https://app.forestadmin.com/Slite/Production/Support/data/organizations/index?filter=%7B%22type%22%3A%22or%22%2C%22conditions%22%3A%5B%7B%22operator%22%3A%22is%22%2C%22value%22%3A%22?q=${query}%22%2C%22fieldName%22%3A%22id%22%2C%22subFieldName%22%3Anull%2C%22embeddedFieldName%22%3Anull%7D%2C%7B%22operator%22%3A%22is%22%2C%22value%22%3A%22?q=${query}%22%2C%22fieldName%22%3A%22domain%22%2C%22subFieldName%22%3Anull%2C%22embeddedFieldName%22%3Anull%7D%5D%7D`;
popclip.openUrl(url, {app: 'app.loshadki.OpenIn-setapp'});
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)
If you’re using the Setapp edition of OpenIn you’ll want the bundle id app.loshadki.OpenIn-setapp instead.