If it is a page that is not using a popup layout, you could create a snippet with all the content of your page and create a second one showing this snippet using a popup layout.
change the layout of the page to PopUpLayout, and add a Show Page activity to the microflow pointing at the popup page.
If the page you open was created as a pop-up page, that's how it will open.
Hi There I have the same problem,
How can i add an full pop up page, which comes before the login page, like an sort of Cookie page, but then a Pop up page?