Microflows are server side and could not open a link.
Though a widget or nanoflow can help you out. There is an “Open URL” action in the Nanoflow commons https://appstore.home.mendix.com/link/app/109515/
You need to get the microflow to open a page with a URLRedirector widget on it first. This will then redirect the browser to the new URL. You can download the widget from the App Store.
Hope this helps.
You can trigger a nanoflow to open a link using
window.open(url, windowName, [windowFeatures])