background.js

chrome.declarativeContent.onPageChanged.removeRules(undefined, function() {
    chrome.declarativeContent.onPageChanged.addRules([{
        conditions: [
            new chrome.declarativeContent.PageStateMatcher({
                pageUrl: {hostEquals: 'teams.microsoft.com'},
            })],
        actions: [new chrome.declarativeContent.ShowPageAction()]
    }]);
});

/**
 * taken from https://github.com/robol/save-participants-extension by Robol
 */
chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
        if (request.action == "download") {
            let msg = request.message;

            // Format the date to be appended to the filename
            let now = new Date();
            let filename = 'teams-chat-' +
                now.getFullYear() + '-' +
                (now.getMonth() + 1).toString().padStart(2, "0") + '-' +
                now.getDate().toString().padStart(2, "0") + '.txt';

            var blob = new Blob([msg], {
                type: "text/plain"
            });
            var url = URL.createObjectURL(blob);
            chrome.downloads.download({
                url: url,
                filename: filename
            });

            sendResponse({
                status: 'completed'
            });
        }
    }
);