Let the site admins know through a PM, although more than likely they'll chime in here.
While I run adblocking suites on browsers, disabling them doesn't seem to give me any popups, although PiHole may also be blocking them but I doubt it is.
If it only happens on this site that's something to address to the admins or perhaps the site owners, but if it's on other sites too it's likely a pesky browser extension to full blown malware.
Can you give any details to recreate the popup?