1. Inside your harness, in any section add a button control and give any Test ID (like I have used "ScreenLoadModalButton"). Then add an event as Click and action as Launch Action. Mention the flow action which will be shown as modal popup on the screen. Make Visibility of the button as Never and Reserve space when hidden as checked. Else you can use visibility:hidden; to make it not visible on screen.
2. Create a js file with the following piece of code: