Try reading the documentation for writing a Chrome extension. The event you're looking for is
chrome.webNavigation.onCompleted
:
chrome.webNavigation - Chrome Developers[
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You will need to add the relevant permissions to your extension's manifest.
This SO thread has more details:
On page load event in Chrome extensions - Stack Overflow[
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NB: An extension that pops up a new window when a page is loaded is likely to annoy your users. Depending on what the popup does, it could even be seen as malware.