Hey Peter, I totally agree with Pim on the widget he suggest, that one can be found in the appstore here. Only it isn't possible to place content to the clipboard by a microflow. A microflow is executed on the application/server and the server can't address the pc of the user. Only the client/browser can do that, that's why the widget.
You need another construction. You should already have the content present for the user when he opens the page with the copy to clipboard widget, instead of generating it when he clicks the widget.
Check out this widget: https://github.com/mendix/CopyURL it's the same used here at the right of the topic.