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.
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 can trigger a nanoflow to open a link using
window.open(url, windowName, [windowFeatures])