I'm looking for a way to determine the tabId in the content script, so I can send messages from content to popup.

Now, I might be wrong here, but I get the impression it cannot be done. The best solution I've seen so far is this

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    console.log("sent from tab.id=", sender.tab.id);
});

But in my case no messages are send, so how can the content script send the first message. As far as I know, for the content script to send messages to the popup window the tab-id is needed.

What I'm trying to do here is that the content script sends a message to the popup window when the page is loaded and ready