In your Overlay Editor, click the Launch button and select Copy your overlay URL. Then on XSplit Broadcaster, go to Sources > Webpage… and paste the unique Overlay URL. Click OK.

To stretch your overlay to fit your screen, just right-click the source to open the Properties window. Under Display > Resolution, make sure Custom is selected and then set the Width to 1920 and Height to 1080.

You can also refer to this video for help: